Here's another TED-Ed brain bender that can ruin your evening. I actually tried for a little while to solve the puzzle on my own, but I got bogged down and went for the answer. Yeah, there's math involved, as well as logic, which is why I got bogged down.
Even if you don't try to figure it out by yourself, the story turns out to be kinda cute, with endangered species, sharks, and a submarine. -via Laughing Squid