The helicopter blades in the video above appear to be stationary, but if you understand the science behind it, you’ll realize this is just a clever optical illusion. That’s right, the blades look the way they do because the camera shutter speed matches the helicopter’s rotor. “Imagine you’re shooting a 24fps video of a helicopter rotor that spins one full rotation every second. In the video, each rotation will thus be broken into 24 frames. You’ll see the blades rotating normally, just moving one-24th of their full rotation each time. But, if the blades spin exactly 24 times each second, and you’re still shooting at 24 fps, each full revolution will be represented by only one frame. The blades will arrive back in their starting place each time the camera captures a frame, so they’ll look like they’re standing still,” according to HowStuffWorks. Click here to view the first image in today’s viral picture gallery. Continue reading for the five most popular viral videos today, including one of molten copper being poured on a rubber band ball.