X is for xylophone or toys that make sounds.
When I say xylophone I really mean the battery powered contraption that is mostly out of tune but will put forth sounds in frequencies close to the established notes. I'm not sure they make real ones for babies. I certainly don't remember playing with one.
I don't understand thefascination it holds for babies but then again I was one a long time ago. Memories don't go that far back.
Actually, at this point I only remember wincing at the sounds while my baby was absolutely fascinated by it. Perhaps it's time to introduce her to a well tuned guitar.