I used to play piano by ear.

This joke is a delightful example of wordplay, specifically a pun, that hinges on the double meaning of the phrase "playing by ear." The humor comes from the sudden, literal interpretation of an idiom. Initially, "playing piano by ear" refers to the musical skill of reproducing melodies and harmonies solely by listening, without reading sheet music. It suggests an innate musicality and the ability to spontaneously create or imitate music.

The punchline cleverly twists this well-understood musical concept. Instead of describing a method of musical learning or performance, it suggests the absurd image of someone physically using their ear to press the piano keys, implying a switch to the more conventional (and practical) method of using hands. This unexpected literalness creates a comical disconnect, as the listener's brain shifts from the abstract, idiomatic meaning to a silly, concrete image.

The idiom "playing by ear" has roots in the 17th century, first appearing in John Playford's 1658 handbook, A Brief Introduction to the Skill of Musick, where it referred to learning "without book". Over time, it also developed a figurative meaning of making decisions without a strict plan, simply responding to circumstances as they unfold. The joke, however, sticks to the musical origin, playing on the contrast between the skilled, auditory approach and the obvious physical action required to play an instrument.