The Key.

I have a toy musical box that used to be Alexandra's.  She wanted to get rid of it but I decided to keep it for Isabella.

I went to open it, and found I couldn't.

'You've locked the key inside', Alexandra told me.  Oops!

I fiddled around, tried to pull the lid.  Put a knife under the lid to try and release the lock.  Nothing.

I sat there looking at it.  It's for a child, it must be easy to open it.  I pressed the keyhole and ping! it opened, and there was the key.

Easy when you know how.