This is a hard haiku constraint because THUMB is concrete but tonally unstable: it can become gesture, injury, evidence, punchline, or measurement depending on the setup. That gives writers options, but it also means the last line can feel arbitrary unless the poem builds a clear function for the word. Mick’s “Who’s to BLAME, they ask / We missed by seven CUBIT / The culprit? A THUMB” is a good example of handling that well: the constraint words are folded into one compact logic of blame and mismeasurement, so THUMB arrives as the key unit of error rather than as a generic body-part ending. Across the set, the stronger entries make the target word do real work inside the poem — sign, bruise, print, joke, or measure — which is what keeps the haiku from collapsing into simple naming.