Light. You just KNOW that it's going somewhere, fast. It's a known scientific fact* that It can be very difficult to catch photons traveling at 299,792,458 metres per second. Additionally, if one allows one's brain to get twisted and confounded by wave/particle duality, it's difficult to recall the optimal shutter speed in tricky situations. Let's do this project anyway, yes? Go on then---shoot light in motion-any light, any kind of motion. The key is that the image must clearly show light in motion.