Credit Card check

One of the paid sites wanted to do a credit card check. Since I was reluctant to share this info, i just came up with a number and typed it in. Incidentally, the system recognized that the number was fake even before i pressed the submit button.

Amazed, I checked out to find that the credit cards follow a Luhn Algorithm to preprocess the number before submitting it to the VISA/MaterCard/Amex.