I just dont get How is it done?
http://www.regiftable.com/regiftingrobinpopup.html

No matter what number you pick, when you subtract the two digits, you end up with a multiple of nine.

18-1-8 = 9
12-1-2 = 9
54-5-4 = 45
99-9-9 = 81

It won't matter which two digit number you chose.
You will find that *all* the gifts that are the same as your gift are multiples of 9.

Ahhhhhhhhhhhhhh!! I see. Thanks!

This is actually a convenient side effect of base ten. This works with a number of any length, not just 2 digits.

Similarly, if you take any x digit number then rearrange that number in any fashion, their difference is a multiple of 9. I.e. 54321 - 31254 is divisible by 9.