Ever wonder how a website knows your credit card number is "invalid" before you even hit the submit button? In this commando-style holiday episode, Pete and Noah dive into the secrets of Luhn's Algorithm, a clever approach that uses simple arithmetic to validate sequences of digits.

• Leave us a voice message

• Find us on Twitter

• Send us an email

Questions Answered in the Episode: • How does the Luhn algorithm work to validate credit card numbers? • Why do credit card companies use a checksum digit?