Take any 4 digit number.  Create a second 4 digit number from the first one by reordering the
digits.  Then subtract the smaller number from the larger one.  

Pick a digit from the second number, but do not choose a zero.

Tell me the other three numbers and I will tell you what number you choose.

How do I do this?

Try it out at  
http://digicc.com/fido/