When your phone takes a photo of something, it scans the frame of view line-by-line from top to bottom quickly. But, if you're photographing an object like a fan or plane's propellor that's moving very quickly, the scanning exposure can warp the final image. That's the rolling shutter effect. Using high-speed camera footage to simulate the warping, Smarter Everyday shows us exactly how the rolling shutter effect occurs. The guitar strings are the coolest; more of that in this video:
P.S. Here's the behind-the-scenes for Smarter Everyday's rolling shutter video.