Transcript:

So earlier today, I posed a puzzle. I'm going to reframe it here. It's a closed shape with four sides in two dimensions. Closed four-sided shape in two dimensions. Mutually the sides intersect at right angle. That's it—closed four-sided shape in two dimensions. Sides intersecting at right angle. So there are four right angles. What is the shape? It's not a square. It isn't a square. What is the shape? 

Pause it here and solve it if you can, because I'm going to reveal the solution now. 

This is the solution: So, take a unit circle and cut out a pie section of it. So from the center, there are two lines going out and that part is cut off and extend those two lines out. So L1 is the first side, L2 and L3 are the second two sides, so three sides. L4 is the arc of a circle of radius L2 plus one. And that's the answer. It's pretty interesting.