Imagine Bitcoin is a special treasure chest with 21 million gold coins inside. The person who made the chest put a magic lock on it that nobody can ever break.

You can earn these coins by solving math puzzles ("mining"), but the magic lock has a rule: Every time 210,000 puzzles are solved, the reward gets cut in half.

So, first you got 50 coins for a puzzle, then 25, then 12.5, and so on. If you keep cutting the reward in half forever, you eventually can't get a full coin anymore. If you add up all the coins that will ever be given out from these shrinking rewards, the total is exactly 21 million.

No one can ever add more coins because the magic lock's rule can't be changed.