From the Super Mario Wiki
The Glissando Piano is one of the musical instruments from WarioWare: Twisted!. It is at the very far left of the instruments category. Its icon is a black piano. The Glissando Piano is basically a regular piano (the grey piano), but instead of having the player's finger stuck on one key, the player can glide through all the keys without letting go of A. There are fifteen usable keys. The ten black keys cannot be used.