What does "dumb as a box of rocks" mean — and why is it funny?

informal, humorous

Meaning

Extremely stupid.

Where it comes from

A modern American comparison. Rocks are the very picture of something inert and thoughtless, and a whole box of them doubles down on the idea.

Why it is funny

The humor is the sheer flatness of the comparison. A single rock would make the point, but the phrase insists on a whole box of them, as if one rock were not quite stupid enough to capture the situation.

Used in a sentence

"The plan was dumb as a box of rocks, and somehow it worked anyway."