What is made of glass?

My youngest learned to ride his bike early, always eager to catch up with his older brother. I wan't long before the inevitable happened, his foot slipped of the petal and he landed on the top bar of the bicycle. We have all been there! I rode over to see if he was OK, he looked up at me with a stricken look on his face and asked, "daddy, are testicles made out of glass?"