How does this magic trick work?

one of my friends she layed out ten cards and gave us a tiny piece of paper and told us to put it under any of the cards and rearrange it if we like, and she left the room and there was no way she can look in and we put it under one of the cards and rearranged it a bit. She came back walked around a bit and touched our heads to "mindread" us. Then she figured out whcih one it was in, then we said the cards were to small and the one with the paper underneath woudl pop up so we dit it with ten magazines and we used a normal sized card and put it inside, again she knew which one it was in! Then we told her to leave and we put it in another magazine that wans't part of the ten that was just lying around, she still knew it was in it! then we didnt' put it in at all and she knew we didn't put the card in. How did she do it????