Here's a neat little trick I just learned: Tell your friend to write down a 3 or 4 digit number, but don't let you see it (it can actually be any number of digits over 2, but if your friends are like mine they get confused easily). Tell them to use a variety of different numbers. Let's say our friend picks 1234. Now tell them to rearrange those digits into a new number, like 4321 or 2143 or 3241 etc, again don't let you see it. Let's say they pick 4123. Now tell them to subtract the small one from the big one. 4123-1234=2889. Now tell them to circle one of the digits in their answer, but don't circle a 0, cause it's already a circle. We'll say they circle the 9, tell them to take the uncircled digits and rearrange them and tell you that number, 288 or 828 etc. and you will tell them what digit they have circled. Someone want to try? Then I'll tell you how it's done, although once you look at it closely you'll probably figure it out. It is pretty neat though.