Piano is one of the hardest instruments...
Piano is one of the hardest instruments to pick up.
This is a pun. Colloquially to "pick up" a skill is to learn it, however literally to pick up something means to take it into your hand and raise it up from where it was sitting.
Pianos, being large, are probably the hardest musical instrument to lift up off of the ground, however it may or may not be true that it is hard to learn how to play it.
This joke could be pretty stealthy if you delivered it right.