Hollow-Sounding Head

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.

Anime & Manga

 * Baikinman from Anpanman is like this.

Commercials
"Draught Guinness: Just so you can tell us apart, he sounds different.
 * 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.

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."

Film

 * Combined with In One Ear, Out the Other in National Lampoon's Loaded Weapon 1. When Emilio Estevez romantically blow's 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.
 * There is a great scene in The Gendarme and the Extra-Terrestrials where the titular policemen are trying to prevent a type 2 Alien Invasion. Their only clue is that they are made of metal and so their whole body sounds hollow if you strike them. Panic quickly strikes when the characters discover that their own heads also all sound empty, which ends when the chief tests his own head and, upon finding it just as empty, declares that all is well.
 * In the second Teenage Mutant Ninja Turtles movie, Donatello knocks on Michelangelo's head and it produces a hollow clonk.
 * "Hmm not quite ripe yet..."

Live Action TV
"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.
 * 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 Chapulin Colorado involves El Chapulin and the Victim of the Week guessing about the Secret Passages of an old house. Cue this gag:

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]."

Literature

 * Unusually used as an actual plot point rather than (just) a brief gag in Charlie and The Chocolate Factory. In both the book and the second film, Veruca Salt is tested by squirrels to see whether she is a 'bad nut', bad nuts sounding empty when tapped. A squirrel taps her on the forehead, and when this produces a hollow sound, they dump her down the garbage chute with the other 'bad nuts'.
 * 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.

Video Games

 * In the first Quest for Glory game, 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.

Western Animation

 * 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 Heifer — not because Heifer's normally stupid, but because he's become non-responsive. Rocko and Filbert realize that this is because Heifer'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.
 * Played with soon after Quilt is introduced in Dominic Deegan. Only his head doesn't sound hollow...

Real Life

 * Open your mouth and tap on the top of your head.
 * I find that opening my mouth is not necessary. Air pockets?