A dad joke is a small machine. Setup, hinge, drop. The setup loads a word with one meaning; the punchline catches it doing another job entirely. Today’s six all run on the same trick — the velcro that’s a rip-off, the step ladder you never knew, the paranormal jeans, the beaver who was nice gnawing you, the surgeon who said knock yourself out, the arthritis that hurts pi out of ten. Strict economy, no flourish, the audience does half the work.