Mathematics
Law of Large Numbers
Quick fact
The law of large numbers was first formalized by Jacob Bernoulli in his 1713 work Ars Conjectandi; it is the mathematical guarantee that insurance companies can predict losses despite unpredictable individual claims.
Why this is interesting
When you flip a coin many times, the percentage of heads gets closer and closer to 50%—but why doesn't a streak of tails guarantee a heads next time?