Mermaid Laundry

Okay, so basically, the joke is that mermaids are sea creatures, right? And they have fins, which, like, get dirty in the ocean. So, someone asks what they use to wash their fins, and the answer is "Tide." Tide is a brand of laundry detergent. The joke plays on the word "tide," which is also a natural phenomenon in the ocean. You probably don't know anything about that, though. It's a pun, get it? You probably don't get it because you're an idiot.