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Condensation Clue

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A classic way for an investigating character to discover a hidden message is when it's been written/drawn invisibly onto a window or mirror with someone's bare fingertip. Imperceptible under dry conditions, such a message will be revealed if steam is allowed to build up near it, causing condensation to fog up the glass everywhere except where fingerprint-grease adheres to its surface.

Occasionally used to convey a message between two characters who are present at the same time, but either can't or won't speak aloud. In such cases, grime on the glass can serve the same purpose as condensation. Also occasionally seen in a Haunted House, presumably because ghosts have an easier time manipulating condensation than writing implements.

This trope is commonly used in bathrooms, where mirrors are found in close association with showers and sinks as sources for steaming-hot water. Only rarely will any marks besides the crucial clue appear, which can raise Fridge Logic issues if the surface in question hasn't been cleaned since it was last handled, so logically ought to have previous (random) finger-marks on it.

A family-friendly sister trope of Couldn't Find a Pen. An Ice Person may use frozen condensation to leave messages this way, etching them in frost. Often a subtrope of Invisible Writing, and a sister trope of Writing Indentation Clue.


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  • One promotional clip from Bristol Myers Squibb features people huffing on windows to create fogged-over patches, in which questions about lung cancer appear.
  • A horror-themed Taco Bell commercial, staged as a film trailer for "The Craving", concludes with a message that nacho fries are back on the menu. It's scrawled on the fogged-up rear windscreen of a parked car, as if by an unseen occupant of the back seats.

    Films — Animation 
  • In Rise of the Guardians, otherwise-imperceptible Jack Frost provides Jamie with a sign that the Easter Bunny is for real by frosting over his window and drawing an Easter egg in the frozen condensation.
  • In The Simpsons Movie, after the Simpsons escape the dome, Colin breathes on the glass inside and writes down a musical staff with notes, dubbed "Lisa's Song", which Lisa then vocalizes.

    Films — Live-Action 
  • In Brightburn, Brandon marks the diner's frost-covered windows with his double-B symbols before he attacks the proprietress. The frost melts by morning, but the sheriff discovers the markings by huffing on the glass.
  • Constantine (2005): Constantine is sure that Isabel Dodson didn't commit suicide without leaving a message for her sister Angela. He uses a repeated Armor-Piercing Question on a reluctant Angela to force her to admit that she and Isabel used this technique as children. Angela then breathes on the window in Isabel's room, revealing her final message to her sister.
  • The heart the daughter drew on the window in Flightplan (2005) confirms that Jodie Foster's character isn't going crazy and hadn't just imagined that her supposedly dead daughter was on the plane. Possibly an Inspiration Nod to the very similar plotline in The Lady Vanishes.
  • In The Fog (2005), one of the four teens whose boat is first to fall prey to the haunted fogbank goes into a trance and draws a set of scales on the window. Presumably this is to foreshadow the ghosts' motive of payback for their murders at the hands of the town founders (scales = justice).
  • In Ghost (1990), Sam uses this against Willy, the man who murdered him. After terrorizing him by trapping him in an apartment and causing objects around it to hurl themselves at him, Willy hides in the bathroom, only for Sam to steam up the bathroom mirror with hot water in the sink, then write the word "BOO" on the glass.
  • In Harry Potter and the Half-Blood Prince, Lavender huffs on the window of Ron's train compartment and draws an overly-cutesy heart in the condensation.
  • At the end of I Know What You Did Last Summer, Julie finds the words "I still know" drawn on a fogged-up shower stall, presumably put there by the killer to bait her close enough for him to crash through the glass and attack her.
  • James Bond's contact Luigi Ferrara leaves a message for him in his hotel room using this method in For Your Eyes Only.
  • In Kill Bill Pt. 2, the swordsmith is unwilling to say Bill's name aloud, so he writes it in condensation on a fogged-up window for the Bride to read.
  • Miss Froy in The Lady Vanishes writes her name with condensation on the train window. When the heroine sees it again, it confirms to her that she's not crazy and something is amiss.
  • Used to explain a clue in Murder on the Orient Express (1974), when Poirot writes the symbol "H" on a fogged-up window as he's revealing the monogrammed handkerchief's real owner.
  • In Ouija, one of the mysterious "Hi friend" messages appears on the inside of a fogged-up car window.
  • In the suspense/horror film Patrick, the titular telekinetic coma patient causes the words "You are mine" to appear on the fogged bathroom mirror of a nurse his damaged brain has become obsessed with. When she reacts by yelling how repulsed she is by his psychic stalking, he makes the mirror de-fog and then shatter explosively, cutting her badly with its shards.
  • In the film, Paycheck, Michael Jennings (Ben Affleck) contractually has his memory erased after completing a job, a wipe that has dire consequences. Knowing he'll need to contact his girlfriend Rachel Porter (Uma Thurman) for help after the memory wipe, he writes a message for her to meet him at their favorite bistro onto her bathroom mirror so that she will see it the next day when the mirror fogs from her shower. The Big Bad figured it too, as he had planted bugs in the bathroom, and saw her mood change from depressed to happy. He instructs The Dragon to position himself so that he's at the same angle as she was, and seeing that she was looking at the mirror they both realize where the message is.
  • In Slumdog Millionaire, the show host writes the letter B on the steamy bathroom mirror for Jamal to notice. It was supposed to be the answer for the next show question but as it turns out - the clue was wrong. Good thing Jamal didn't fall for it.

  • One of the ghostly behaviours that turns up is a threatening mirror message in The Dark Beneath The Ice.
  • In The Demon's Grave Nora gets an unsettling message on her bathroom mirror from an unknown entity near the beginning of the book.
  • Used in an Encyclopedia Brown story where two spies in a hotel pass information to each other without being caught. The first took a shower and drew the info on the mirror, then waited until it faded to leave (he opened a window to speed the process). The second went in and took a shower, copying down the message as it reappeared on the mirror, then wiped the mirror down so no one else would notice.
  • Forest Kingdom: In the Hawk & Fisher spinoff series' book 2 (Winner Takes All), a political candidate's hired sorcerers deliver a death threat to his opponent James Adamant via this trope, magically causing a window to fog up and then "drawing" a leering, speaking face in the fog with the tracks of dripping water, followed by a telepathic whisper.
  • How to Survive a Horror Movie cites a ghost's writing "Get out!" in the fog of your bathroom mirror every damned day as a sure sign that said ghost is of the "Attention Whore" type. "Yes" might also appear, if Death has accepted your offer to do The Grim Reaper a favor so your preordained demise will be deferred, but only if the writer can't come up with anything more original.
  • In one of the Johnny Maxwell Trilogy books, Wobbler writes a note on a fogged-up window so he won't forget it, then keeps rushing back to huff on the glass and keep it foggy so the message remains visible.
  • In King Rat, the title character writes a short message to the protagonist in the grime on a window when he's sneaking him out of jail, as speaking aloud would be overheard by the nearby police.
  • References in Lost by Gregory Maguire, in which a skeptic claims that seeing words appear on a fogged-up window would only make him marvel at the statistical unlikelihood of the condensation dissipating in that pattern, not leap to the conclusion that it's the handiwork of a ghost.
  • Played with in Kim Newman's "The Man Who Got Off the Ghost Train", when Richard huffs on the mirror in his luxury train compartment's washroom. The fog of his breath reveals an ominous message in the condensation ... which his female associate drew on it to troll him and see if he'd notice.
  • Repairman Jack: In The Haunted Air, a ghostly presence writes on a fogged-up mirror to communicate with Lyle in real time.

    Live-Action TV 
  • Buffyverse:
    • In an episode of Angel, Spike has been reduced to being a ghost and can't be seen and can barely interact with the world. He manages to write a single word in the condensation while Fred showers.
    • Used on Buffy the Vampire Slayer when Faith entices Buffy into ditching school to go slaying by breathing on the classroom window, then drawing a heart and 'staking' the middle of it with her finger.
  • On The Closer, the episode "Products of Discovery" reveals this is the method the gang leader used to communicate a hit on a witness testifying against him.
  • CSI: NY had this in "Unspoken"...the villain opens fire at a political rally and though no one is hit, he realizes Lindsay saw him. He goes into her hospital room wanting to kill her and writes 'I'm sorry' on the window. He doesn't kill her, because he realizes she's a mother and he has a big thing against hurting kids, but the next morning, Lindsay's husband, Danny, is standing by the window with his coffee, and the steam reveals the would-be killer's words.
  • In CSI: Miami, they use this principle to reveal shoe prints after a woman is electrocuted while taking a bath.
  • Doctor Who:
    • Disguised as a cleaning lady in "The Green Death", the Doctor discreetly signals Captain Yates by soaping up a window, writing a message in the suds, then wiping it away.
    • In "Pyramids of Mars", when Sarah Jane is trapped in the "Decadron Crucible" on Mars, the Doctor tells her to "R-E-L-A-X" by writing in the dust/condensation on the surface of the tube (for bonus points, he even writes it backwards).
  • Endeavour: In "Game", Morse investigates a suspicious death at the public baths. When he turns on the hot water, the mirror steams up to reveal the word "DENIAL" written on it.
  • On Ghosts, Trevor writes a message on the fogged-up bathroom mirror to let Jay know that Sam lied about the ghosts' not wanting to play D&D with him. The effort of doing so nearly kills Trevor, at least figuratively speaking.
    • In the second Halloween episode, Sam gets locked into the ghost-proof (and air-tight) secret vault. Jay realizes the previous trick, fogs up the mirror and waits. Unfortuately, Trevor tries to by succinct, writing "SAM SAFE." Jay mis-interprets until Trevor changes it to "SAM in SAFE".
  • In Ghost Wars, the main character actually watches the words "No Hope" manifest eerily on a mirror. In this case the condensation itself manifests from nothing, conveying the impression that a ghostly presence is making the mirror colder in a humid room, everywhere except the lettering.
  • Adelaide finds a message written on glass in a window overlooking a steamed-up laundry in the pilot episode of Houdini & Doyle.
  • In "Assignment 2" of Sapphire and Steel, a series of 11s appears repeatedly in the grime on a hotel room window, indicating the exact date and time of Private Sam Pierce's death.
  • Supernatural. In "Of Grave Importance", a ghost Bobby Singer gets increasingly frustrated that all his efforts to contact the Winchesters are ignored or dismissed. He finally succeeds with this trope.
  • Used frequently in season 2 of Teen Wolf. Scott is still seeing Allison even though her father has forbidden it (at gunpoint!), so he leaves rendezvous meeting times on her car window for her to reveal with a huff of breath when she's in her car alone.
  • A message revealed by steaming a mirror provided a clue on the pilot episode of Whodunnit.

    Video Games 
  • In Alice Is Dead, a mirror in the nightclub has a hidden message that can be revealed by messing with the club's air conditioning.
  • The passcode to the keycode door in Another Code can found by blowing into the mic, causing the window to fog up and reveal the code.
  • Azada:
    • Turning on the hot water in the bathroom reveals which two symbols to click on to open a locked chest.
    • In Azada: Elementa, a figure behind some French doors you need to open writes a warning on the doors' fogged-up glass.
  • In Bioshock 1, Andrew Ryan, in trying to stop you from reviving Arcadia, gasses Julie Langford in her lab, and just before dying, she scrawls the code to her safe on the window.
  • In chapter 2 of The Cat Lady, Susan figures out the fellow patient's mother's name by leaving the hot water running under the bathroom mirror, revealing part of the name written on it with a finger. Figuring out other parts requires being drugged.
  • Dark Fall:
    • In Dark Fall: The Journal, you need to re-start the hotel's running water so you can steam up one of the bathroom mirrors and expose a lyric.
    • In Dark Fall: Lost Souls, Verney's ghost writes a clue in the grime on a different bathroom's mirror.
  • In Morpheus, turning on the steam room's taps fogs up the door to reveal a bible verse, which doubles as Grace Thermon's stateroom code.
  • Mystery Case Files:
    • In Escape from Ravenhearst, the push-button combination to a padlock appears on a bathroom mirror once you repair the water heater and turn on the hot-water tap.
    • In Shadow Lake, a series of symbols is written in the condensation on the inside of a car window. A dying man inside the car wipes it away, but you can re-view the cinematic if you miss any of the symbols the first time around.
  • In Resorting to Danger, one of the Dossier sub-series of Nancy Drew games, a password can be revealed on the see-through door of an industrial dryer if you turn it on so its window steams up.
  • One of the clues in the Stonehenge site's RV from The Omega Stone can be revealed in this way by entering the shower stall and turning it on.
  • Used twice in Phantasmat Behind The Mask: once to reveal a compass-based code, and again in the collector's edition to explain the game objective of the epilogue.
  • In Rhiannon Curse Of The Four Branches, an unseen presence writes a clue on the upstairs bathroom mirror when you turn on the hot water and let it steam up.

    Visual Novels 
  • One of the late game tasks in Last Window is investigating another tenant's room for clues as to what he's up to. Running the sink with hot water reveals a passcode in the mirror that you need to open a safebox.

    Western Animation 
  • Arcane. Used for the cliffhanger ending of "Oil and Water". Caitlyn is taking a Shower of Angst, turns off the water and heads to the bathroom mirror to freshen up, only to see the telltale signature of Jinx's monkey sign drawn through the condensation on the glass as she gets closer. Caitlyn can only gasp in horror at realizing she's Alone with the Psycho before hearing Jinx behind her as we Smash to Black.
  • Miraculous Ladybug: When Oblivio erases Ladybug and Cat Noir's memories just before they turn back into Marinette and Adrien, they're left scrambling to try to determine their own identities. They get their names off school ID cards in their pockets, then hit on the idea to use the contacts in their cell phones to call for help. Unfortunately, their cell phones are locked, and neither one remembers how to unlock them. Then Marinette breathes on the screens, which shows the patterns they use to unlock. Good thing they both apparently cleaned their screens, unlocked them, and then did literally nothing else, because they're pristine otherwise.
  • The setup is seen in an episode of The Simpsons where Homer and Marge are trapped in a revolving door that dropped on them from a hurricane. Homer writes on the glass, "Save her first."
  • On Steven Universe, when Steven traps himself and Connie in a bubble-shaped, soundproof magic shield, they both breathe on the sides and draw in the resulting condensation to try and convince Onion to break it and free them. Steven tries to get him to use a harpoon gun with a complicated drawing. Connie just writes "harpoon gun".


Video Example(s):


Monkey sigil in the mirror

When Caitlyn finishes her shower, she realizes she's in trouble when Jinx's monkey sigil appears on the mirror as the steam clears.

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Example of:

Main / CondensationClue

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