Dr. Slump: Arale-chan - episode 2 - 14 Apr, 1981

Everybody in Penguin Village Junior High is asking Arale to join their clubs but Arale seems too good for all of them. Arale comes home from school and tells Senbei that the girls at school said that she is missing a part of the body that the other girls had. Senbei makes a pair of glasses that see through peoples clothes so he can see girls naked and "find out what she is missing". In the end it actually was a belly button that she was missing. (Source: Wikipedia)