The riddle
I Have a Key but No Lock. What Am I?
The longer form
Most tellers say more than one line: "I have keys but no locks. I have space but no room. You can enter, but you can't go outside. What am I?" The answer is a keyboard. Its keys are the buttons pressed with the fingers. The space bar is a long key with no room in it, and the enter key is for entering, though nobody can walk in. A full-size keyboard often has 104 keys, and not one of them fits a lock. The top row of letters on most English keyboards starts Q, W, E, R, T, Y, which gives the layout its name.
Where a piano comes in
A piano is the other answer people give, and it fits the short wording well. A piano has keys that open nothing. It does not fit "space but no room" or "enter", so for the long form the keyboard is the better match. A close cousin of this riddle asks what has keys but can't open locks.
Why the top row is called QWERTY
The standard English keyboard layout takes its nickname from the first six letters of the top row of letters, Q, W, E, R, T and Y. The name is a plain description of that row, read from the left.
Telling it in order
Give the lines one at a time and pause after each. By the third line, many listeners are already looking at the keys under their own fingers.