Another "play with numbers" website http://digicc.com/fido/ It's probably related to the fact that we have a base-10 number system, but I fail to see how it works... This website does the following: 1) You write down a 3 or 4 digit number, say X=x1 x2 x3 x4 (where the x's denote the digits). 2) You randomize the digits and obtain another number, say Y=x3 x2 x4 x1. 3) You subtract the largest from the smallest one. (call this one Z=z1 z2 z3 z4) 4) You select one of the digits from this number (say z3) and write the remaining numbers in any order, say z4 z1 z2 in my example. 5) Somehow, the website then tells you what the selected digit was (the z3 in my case) based only on this remaining number. Can someone explain in some more detail what the trick is?