Headcanon that Katsuki, when he eventually lets himself relax around people, finds “intellectual jokes” absolutely hilarious. Historical jokes, Science jokes, Mythology jokes, just really nerdy jokes in general. He’s a very smart boy and easily retains a lot of information, so it seems like that’s the kind of humor he likes. (along with slap stick but he tends to be more mean about that and trying to keep up a reputation)
And of course Izuku knows this, because of course, but he also kind of enjoys these jokes, or at least making them. So maybe he tells Ochako a joke and she and Iida and Todoroki all stare blankly and Izuku huffs and says “Fine, let me tell it to someone who’ll appreciate it. Hey, Kacchan!!” and everyone thinks he’s crazy, even Katsuki is beginning to say “fuck off, Deku” but Izuku cuts him off with “What kind of fish is made of two sodium atoms???” pause. “A 2Na!” another pause then suddenly Katsuki has to slam his face into his arms to avoid absolutely breaking with laughter.
Follow Up: here