A model skull complete with eye sockets, mouth, a nose and hair that is forged out of clay. Once fired, this clay head turns into an unbreakable head, if and only if, it can withstand a fall from a second story window or higher. Most unbreakable heads are made in high school art class.
Duder 1: "Whoa dude nice head."
Duder 2: "Yea man it's hella hard now. Wanna throw it out the window and see if it breaks?"
Duder 1: "Of course!"
(after the test...)
Duder 1: "Nice! He's still intact. Let's call him unbreakable head!"
Duder 2: "Solid doggy!"