Why Every CAPTCHA Is Ambiguous

Technical writer Paul Stregevsky articulates what I’ve long wondered:

“Select each [CAPTCHA] square that contains a[n] ___.”

If a CAPTCHA instructs, “Select each square that contains a bus,” how much of a bus must I see? A tire? A taillight?

Why write “contains a bus” when it’s so easy to write “contains all or part of a bus?”

Better yet, why not tell us how much of a bus we need to see?