Here's a little lunch-time fun—Derek Muller of Veritasium poses a puzzle to people hanging out at the beach. It's trickier than it seems. (I have to admit, I followed the same answering pattern as the people in this video, and kicked myself just as hard when the answer was revealed.) I encourage you to pause the video and think it through—think about your answer, and how you might test it. Have a look:

And here's a less-edited version focusing on a single couple's process in figuring it out. Not as fun in some ways, but interesting to see how they process the problem and focus on re-testing their own presumed-correct hypothesis over and over:

(Via Devour.)