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Condensation Clue
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.

Examples:

Film
  • 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.
  • 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.
  • Constantine. 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.
  • James Bond's contact Luigi Ferrara leaves a message for him in his hotel room using this method in For Your Eyes Only.
  • The heart the daughter drew on the window in Flight Plan confirms that Jodie Foster's character isn't going crazy and hadn't just imagined that her supposedly-dead daughter was on the plane.

Literature
  • 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.
  • 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. The second went in and took a shower, copying down the message as it reappeared on the mirror.
  • In The Haunted Air, a ghostly presence writes on a fogged-up mirror to communicate with Lyle in real time.

Live-Action Television
  • 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 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 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.
  • A message revealed by steaming a mirror provided a clue on the pilot episode of Whodunnit?.

Video Games
  • In Azada, turning on the hot water in the bathroom reveals which two symbols to click on to open a locked chest.
  • 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.
  • 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.
  • 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 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.
  • 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.
  • 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.
  • 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.

Western Animation
  • The setup is seen in an episode of The Simpsons where Homer & 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 a girl 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.
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