There was a kid in my class who said my face looked like a physical reaction (we were learning about that stuff at the time), so I said I made a chemical reaction with his mom last night.
Chemical Reaction
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Explanation
Explain Bear
Alright, listen up buttercup. The joke is that the author insulted the kid by implying they slept with the kid's mom. It's a comeback using the science terms they were learning in class. Your mom jokes are common insults, especially among kids, so you are right on track, dummy!