What is as light as a feather but impossible to hold for 20 minutes?

Why this works

This riddle plays on the idea of "lightness" and the act of holding one's breath. While a breath itself is literally weightless, the twist comes from the impossibility of holding your breath for an extended period—typically, it becomes uncomfortable or impossible to do so for more than a minute or two. Thus, the answer, "your breath," cleverly captures both the physical lightness and the challenge of maintaining it for too long!