A teacher hands out busywork: add every number from 1 to 100
As the story goes, a schoolmaster wanted to keep his class quiet, so he set them a tedious task: add up every whole number from 1 to 100, longhand. He expected it to take the rest of the lesson. One student, not yet ten years old, had the answer in well under a minute, without adding a single pair of numbers the slow way.
Reveal the answer
5,050, reached instantly by pairing numbers from opposite ends: 1+100, 2+99, 3+98, and so on. Each pair adds to 101, and there are 50 such pairs, so the total is 50 x 101 = 5,050. The boy was Carl Friedrich Gauss, later one of history's greatest mathematicians; the schoolroom story, recorded by his biographer Sartorius von Waltershausen, is one of the most-told anecdotes in the history of mathematics.