A piano is a large musical instrument that you play by pressing black and white keys on a keyboard. Most people play a piano with their fingers, but Jerry Lee Lewis played with his fingers, feet, elbows, and, ahem, backside.
A piano makes a sound when each key moves a small hammer that strikes a metal string. The inside of a piano looks kind of like a harp. Pianos are vital in many kinds of music, from classical to pop, and in the case of Lewis, boogie-woogie. Piano comes from the original Italian name for the instrument: piano e forte, "soft and loud." Piano is also the musical notation that tells the player that something should be played quietly.