When a character, especially TheDitz, 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 HardHead.

Can sometimes be employed alongside an InOneEarOutTheOther visual gag.
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!!Examples:

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[[folder: Anime & Manga ]]

* Baikinman from ''Franchise/{{Anpanman}}'' is like this.

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[[folder: Commercials ]]

* The people who "Could've Had a V-8" get a DopeSlap 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.

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[[folder: Film ]]

* Combined with InOneEarOutTheOther in ''Film/LoadedWeapon1''. When Emilio Estevez romantically blows into Creator/KathyIreland'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.
* Film/TheThreeStooges 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 AlienInvasion. Their only clue is that the aliens are made of metal and so their bodies sound hollow when you strike them. [[http://www.youtube.com/watch?v=WxndAfkyzAs Panic almost sets when the characters discover that *their* heads are hollow as well]].
* In the second ''Film/TeenageMutantNinjaTurtles'' movie, Donatello knocks on Michelangelo's head and it produces a hollow clonk.
** "Hmm not quite ripe yet..."

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[[folder: Live Action TV ]]

* 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 ''ElChapulinColorado'' involves El Chapulin and the VictimOfTheWeek guessing about the SecretPassages 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 ''[Knoks another panel ofv 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]''.

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[[folder: Literature ]]

* Unusually used as an actual plot point rather than (just) a brief gag in ''Literature/CharlieAndTheChocolateFactory''. 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 ''Discworld/ReaperMan'', when Bill Door complains that the hailstones hitting him on the head make his skull echo. As Bill Door is TheGrimReaper, it's [[DemBones only to be expected]].
* In ''Literature/HarryPotterAndTheOrderOfThePhoenix'', 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.

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[[folder: Video Games ]]

* In ''VideoGame/QuestForGloryI'', a hollow sound will play if you have the hero throw a rock at his own head. But then, [[PressXToDie you did just have the hero throw a rock at his own head]].

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[[folder: Western Animation ]]

* Happens to Homer on ''TheSimpsons'' 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 ''GarfieldAndFriends'', this is how Garfield distinguishes Odie from a set of robotic clones: the ''real'' one's head sounds hollow.
* In an episode of ''TinyToonAdventures'', 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 [[AsideGlance a pained glance at the camera]] and knocks on her forehead, which gives off a hollow metallic sound.
* In one episode of ''RockosModernLife'', Filbert does this to Heffer not because Heffer's normally stupid[[note]]he is, but that's beside the point[[/note]], 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, WaltDisney demonstrates the use of sound effects on cartoons with the help of DonaldDuck. 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.
* Played with soon after [[FrankensteinsMonster Quilt]] is introduced in ''DominicDeegan''. Only his head doesn't ''sound'' hollow...
* 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.
* ''WesternAnimation/TheBeatles'' 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.

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[[folder: Real Life ]]

* Open your mouth and tap on the top of your head.
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