When a character, especially The Ditz, has someone knock on his forehead, we will sometimes hear a loud hollow sound, suggesting that the character's head is empty.
While the knock on the forehead is the most common variation (sometimes preceded or accompanied by an exasperated "Is anyone in there?" by the knocker), anything that produces a 'hollow' sound effect or echo counts.
See also Hard Head.
Can sometimes be employed alongside an In One Ear, Out the Other visual gag.
- Baikinman from Anpanman is like this.
- The people who "Could've Had a V-8" get a Dope Slap accompanied by a hollow sound effect.
- Used in a Guinness beer commercial starring Rutger Hauer as Draught Guinness and Rutger Hauer as Canned Guinness.
Draught Guinness: Just so you can tell us apart, he sounds different.Canned Guinness: (Identical voice) Do I?Draught taps on Canned's head. There is a hollow metallic clang.Canned Guinness: I see what you mean.
- Combined with In One Ear, Out the Other in Loaded Weapon 1. When Emilio Estevez romantically blows into Kathy Ireland's right ear we hear a sound like someone blowing across the top of an empty beer bottle and the hair covering Kathy's left ear billows in the breeze.
- The Three Stooges did this whenever they hit each other in the head (which is often), especially Curly.
- In The Gendarme And The Extra Terrestrials, the titular policemen are trying to prevent an Alien Invasion. Their only clue is that the aliens are made of metal and so their bodies sound hollow when you strike them. Panic almost sets when the characters discover that *their* heads are hollow as well.
- In Teenage Mutant Ninja Turtles II: The Secret of the Ooze, Donatello knocks on Michelangelo's head and it produces a hollow clonk.
"Hmm not quite ripe yet..."
- Dredd has a psychic version during a Battle in the Center of the Mind.
Anderson: Welcome to the inside of your head. It's kind of empty in here.
- In one sketch on The Carol Burnett Show, Carol was a queen and Tim Conway was an inept guard. Harvey Korman, as the king, wanted to know why such an inept person would be assigned to the royal personage. Carol said that Tim got the position through meritorious action in combat — he had swallowed a grenade. She opened Tim's mouth and spoke into it. "Hello...lo...lo...lo...lo."
- One episode of El Chapulín Colorado involves El Chapulin and the Victim of the Week guessing about the secret passages of an old house. Cue this gag:
- Chapulin: this side of the wall is sound [Knocks the wall. Hard wood sound effect] and this panel is hollow [Knocks another panel of the wall. Hollow wood sound effect]. Now, you can try.VOTW: [after knocking the walls] Orale, Chapulin! You really know!Chapulin: Bah, it's all just in the head [Knocks own head. Hollow wood sound effect].
- Deep Space Nine has a variation in "Rejoined", which opens with Dax demonstrating a sleight of hand trick. When Quark can't figure out how it's done (he assumes a teleporter is involved) Dax says it's because his head is full of latinum and pulls a bar of latinum from his ear. When Bashir pulls the same trick, Quark has to shoo away Rom who's peering into his ear to see if there's any more latinum there.
- Unusually used as an actual plot point rather than (just) a brief gag in Charlie and the Chocolate Factory. In the book and most adaptations, Veruca Salt is tested by squirrels to see whether she is a "bad nut" — bad nuts sound empty when tapped — when she tries to claim one of them for herself. A squirrel taps her on the forehead, and when this produces a hollow sound, they dump her where all bad nuts go — down a garbage chute.
- Justified in Reaper Man, when Bill Door complains that the hailstones hitting him on the head make his skull echo. As Bill Door is The Grim Reaper, it's only to be expected.
- In Harry Potter and the Order of the Phoenix, Harry tells aunt Petunia about how Dementors attack by sucking out your soul. Petunia promptly starts shaking her son Dudley and knocking on his head, Harry wondering if she's trying to "hear" his soul rattling inside to be sure it's there.
- Treks Not Taken. In the parody Moby Trek (Moby Schtick IN SPACE!) the protagonist says that just as the captains of old had the sea in their veins, he has the emptiness of space in his head.
- In Quest for Glory I, a hollow sound will play if you have the hero throw a rock at his own head. But then, you did just have the hero throw a rock at his own head.
- Happens to Homer on The Simpsons Halloween episode when the zombies are searching for brains.
- And to Mr. Burns when Homer quits his job to work at the bowling alley and plays the boss's head like a bongo to celebrate. (Lampshaded by Lenny: "Hey, he's gettin' a pretty good sound outta that guy!")
- On an episode of Garfield and Friends, this is how Garfield distinguishes Odie from a set of robotic clones: the real one's head sounds hollow.
- In an episode of Tiny Toon Adventures, Buster is climbing out of a hole in the ground and calls out to Babs that he's "spelunking". When she assures him that there is "a medicine you can get for that", he shoots a pained glance at the camera and knocks on her forehead, which gives off a hollow metallic sound.
- In one episode of Rocko's Modern Life, Filbert does this to Heffer — not because Heffer's normally stupidnote , but because he's become completely non-responsive. Rocko and Filbert realize that this is because Heffer's brain has literally been stolen by the TV and they have to voyage inside to get it back.
- On one episode of the "Disneyland" series, Walt Disney demonstrates the use of sound effects on cartoons with the help of Donald Duck. He shows how the right sound effect can make Donald's head seem hollow when it is hit by a mallet. Donald is not amused, especially when the demonstration moves on to heavier objects.
- An Al Brodax Popeye cartoon had Popeye playing "Yankee Doodle" by knocking on his head and manipulating his mouth to produce the sound of a vibraphone.
- The Beatles cartoon "We Can Work It Out" concludes with the boys agreeing that they'll have good luck as long as they stay together, and, saying in unison "Knock wood!", they knock on Ringo's head.
- In the first season of Total Drama, Izzy says Owen's head is "...good and hollow..."
- In The Adventures of Jimmy Neutron, Boy Genius, Jimmy has an Are You Pondering What I'm Pondering? moment with his Dad and punctuates it by tapping his head. Hugh mirrors the gesture but gets a hollow echo, one that goes on long enough for them both to make a curious face at.
- Open your mouth and tap on the top of your head.