This is something I got from a book, it's a little magic trick, but it's best that you don't tell anyone. :)

This trick is sort of mathematical. 

Material:

5 index cards (or something small) and a pencil or marker.

Preparing the trick:

First, copy the numbers onto your index cards exactly the way they're shown.
Card 1
Card 2
Card 3
Card 4
Card 5
What to do

1) Ask your friend to secretly think of a number from 1 to 31.

2) Give him/her the 5 index cards and ask him/her to hand you each card that has his/her secret number on it.

3) As your friend hands you each card, glance at the number in the top left-hand corner.

4) Mentally add up these numbers.  Their sum will be his/her secret number.

5) Finally, hold these cards to your forehead, close your eyes, and pretend that the cards are speaking to you as you reveal your friend's secret number!