A genre of humor of which I was previously unaware (courtesy of my daughter): elementary particle jokes.

So a neutron walked into a bar and says, "i'd like a beer, please." And the bartender gives him one and the neutron says, "thanks, how much?" and the bartender says, "For you? No charge."

So one ion runs into another ion and says, "Hey, can you help me? I think I lost an electron." And the second ion says, "Wow, are you sure?" and the first one says, "Yeah, I'm positive!"