Saved on this device

Follow your curiosity

What discovery has been shared with you?

FACTREE gives you something interesting to discover every day — and something worth talking about together.

Start with one fact. Explore it, go deeper, then follow whichever branch catches your imagination.

Choose subjects for a surprise

Exploring any topic

Begin your discovery

Your next discovery is one click away.

Choose one or more subjects above, or leave Any Topic selected and let curiosity decide.

Technology

Why Modular Arithmetic Keeps Your Credit Card Safe

Quick fact

The last digit of most credit card numbers is a 'check digit' computed from the preceding digits using the Luhn algorithm. If you swap two digits or mistype one, the algorithm usually catches it, causing the transaction to be declined.

Why this is interesting

Your credit card number isn't just a random string of digits—it contains a hidden mathematical code that helps detect fraud before it happens. How does a simple calculation keep your money safe?