My coworker asked if I wanted to join the company bowling league.

The humor in this joke comes from a clever bit of wordplay and a subversion of expectations. When a coworker suggests joining a bowling league, the natural assumption is that it will involve standard adult-sized bowling balls and a degree of competitive spirit. The setup builds this expectation, leading the listener to anticipate a response related to skill, time, or interest in the game itself.

The punchline, however, pivots unexpectedly to a concern about physical safety, specifically the fingers. While it's true that bowling balls can be heavy and mishandling them can lead to minor injuries, the request for "plastic balls" is where the joke lands. In bowling, "plastic balls" typically refer to the much lighter, often brightly colored balls designed for children or very casual players, not the heavier, custom-drilled balls used in a league setting. The joke exaggerates a common, minor worry to an absurd degree, implying either extreme physical frailty or a comically cautious approach to a seemingly harmless office activity.

This comedic exaggeration plays on the relatable experience of finding bowling balls cumbersome, but takes it to an over-the-top level. The image of an adult joining a company league but only if they can use equipment usually reserved for tiny hands creates a silly and memorable picture, highlighting the speaker's humorous lack of seriousness or exaggerated self-preservation.