ELI5: Explain Like I'm 5
Examples: barcelona, tail recursive program, talmudic jews, speed of sound, how does public key cryptography work?

Coin counterfeiting

Coin counterfeiting is like when someone makes fake money. It is when a person or group of people makes copies of money to try and use them instead of real money. It's against the law to counterfeit money and people can get in trouble for doing it.