A classic close-up magic trick. A magician has someone sign a coin, and then holds the coin below a glass. They tap the coin on the bottom of the glass a few times, and on the fifth or sixth time, the coin passes through the bottom of the glass and appears inside it. Magic!
Only sleight of hand experts can do this one. The coin doesn't travel through the glass - it's very quickly moved from the hand below the glass into the hand above it, and then dropped into the glass.