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.
