A card trick for you based on maths:
Take 21 cards and ask someone to pick a card but not to let you see it.
Tell them to put the card back in with the rest that you have without you seeing where they put it,.....they can even shuffle them if they want to.
Deal the cards, face up, into three seperate piles telling them to tell you which pile their card is in when you have finished dealing.
You must do this at least three times, but each time you pick the cards back up make sure the pile that their card is in is in the middle of the three piles.
Try not to make this obvious.
When you have done this for at least three times tell them that you can show them their card and as you deal the cards out in one pile, under your breath start counting the cards...
The eleventh card will be the one that they chose.....
Always works and it has something to do with maths but I don't know what....
Have fun!