WHAT’S THE DIFF?

What’s so funny about this? We haven’t had a short fun riddle in a while and I think we’re long overdue. I struggled a bit trying to find a suitable background to enhance the context for the joke. I tried to imagine where and when a hippo and a cigarette lighter might appear together for any reason. My first impulse was the jungle; after all, that’s a hippo’s, short for hippopotamus, natural habitat and it’s conceivable that some guy in a uniform could be in a jungle, smoking a cigarette, using a lighter, and meeting up with a hippo - unlikely but not implausible. I also put the scene in a bar, then on the street, but nothing really worked. Then I realized it made no difference. After all, this was a happy dancing hippo and a guy in a weir d uniform with a lighter. The context was irrelevant, so I just put them next to an amusement park because it’s an amusing joke, sort of. The joke is really fun because “hippo” and “zippo” not only rhyme but are only one letter apart, so one answer