[Though this might take someone else off guard, Karamatsu doesn't miss a beat. Instead of saying "You don't know what plastic is?" he's immediately prepared to explain it to her, no questions asked.

Maybe it's because she's a girl that's also acknowledging his presence and talking to him with genuine curiosity, but still.]

Plastic is a...synthetic material, I suppose you would say? Which means it has been chemically created out of other substances. It is quite hard and flexible, durable enough to be used from building structures to clothing accessories!