Rachel’s math club meets in a classroom at the end of a long hallway in her school. This long hallway is lit by a row of 100 light bulbs, each with a chain attached to it that toggles the light bulb on or off when pulled. One day, Rachel arrives much earlier than everyone else, and she sees that the hallway is still dim: no one has turned on the hallway bulbs yet.
Here’s a classic puzzle: Farmer Brown is taking his goods to market: a goat, a wolf, and a head of lettuce. There is a river on the way to the market. Ordinarily there would be a bridge spanning the river. But today the bridge has collapsed, and the only way of getting across is a small rowboat, just big enough for Farmer Brown and one of his items. This poses a problem.