Streetlight Effect

Late one night, a policeman sees a man looking for something under a streetlight. The policeman asks the man what he’s looking for.

“My keys,” he says.

Both of them look under the streetlight together. After a while, the policeman asks the man if he is sure he has lost the keys near the streetlight.

“No, I lost them in the park,” the man replies.

The policeman asks, “Why are you looking for the keys here?”

“This is where the light is,” the man says.

An easy place to search, but not where he should be looking.