With the help of SMBC comics illustrator Zach Weiner, this week Minute Physics tackles the conundrum of why some rocks appear to be naturally round, while others end up flat, perfect for skipping across a lake.

I don't want to spoil the revelation, particularly since it's so entertainingly sketched by Zach this week. But the short answer is it's dependent on the pull of gravity, and whether the rock spends its life rolling about while it's worn and weathered, or if it remains fairly stationary during the process. So go spend 100 seconds getting smarter. [Minute Physics]