Symmetric keys are essential to encrypting messages. How can two people share the same key without someone else getting a hold of it? Upfront asymmetric encryption is one way, but another is Diffie-Hellman key exchange.
Can you save the target from his assassin in a square room with bouncy walls?
Can you stack four colored cubes so that each color only shows once on each side?
Set theory is the foundation of all of mathematics. How does it handle infinity?
