What has keys but can’t open a single lock?

How to think about it

1. Start with the obvious reading: keys open locks. The riddle tells you these keys open nothing, so “key” must mean something else.
2. List other things called keys: the keys on a keyboard, the keys of a map legend, the keys of a musical instrument.
3. Ask which of those has keys as its main feature and is a single object you could point at. A map legend is part of a map, so it feels weak.
4. Between a computer keyboard and an instrument, the classic phrasing of this riddle (“keys but no locks”) points to the instrument, though a keyboard is a fair reading too.

Why “a piano” is the answer

A piano has 88 keys, and every one of them plays a note rather than turning in a lock. The riddle works because English uses the same word for the metal key in your pocket and the black and white keys under a pianist’s fingers. Nothing about a piano can open anything, which is exactly what the question insists on.

Common wrong answers

• A keyboard. This answer is close and many people accept it. The traditional riddle predates computers and is usually told with a piano in mind, so the word “keys” alone is not enough to pick a keyboard over a piano.
• A locksmith. A locksmith has keys and can open locks, which is the opposite of what the riddle says.