1 Old Man: Good day, sir. I'm from an acrobatic troupe called Bodies in Motion. Would you care to see our performance on the morrow?
2 Isaac Newton: Perhaps. But aren't you a little old to be doing acrobatics?
3 Old Man: Oh, I retired from performing years ago, but I still do publicity for the company.
4 Old Man: After all, a Body in Motion tends to remain in Motion.
4 Isaac Newton: Hmmmm...

This is the second half of Newton's First Law, which was covered in the annotation to comic #2295. I tried to make the punchline non-obvious from the first panel, but it was really hard. There's not much other call for the phrase "a body in motion".

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2022-07-18 Rerun commentary: On the other hand, a geriatric acrobatic company might be entertaining to watch, if you're not into thrills and excitement.