If you take a two digit number and subtract from it its two digits, you can only end with certain results. These are:

81
72
63
54
45
36
27
18
9
and 0

If you look at the table, all these corresponding letters with the numbers listed above are the same. Typically most people will be puzzled by this for the first few times they try it. After about 5 tries though, they will start having numbers repeat and will become suspicious.