How did he do this magic trick?

So I have this friend and he did a magic trick and basically what he did is get a pencil top eraser and rubbed it really fast between his hands and he put his two hands up with his fists up and told me to guess what one it was in and I guessed and nothing was in that hand and so I picked the other one and nothing was in that one either....then he rubbed his hands together again and the eraser appeared. It was really cool and I asked him to do it again but he said no-_-......so does anyone know how he might've done this trick?
Thanks