First you're taught that light is wave. Then you get a little older and your teacher explains that it's actually particles called photons. Wait, which is it then? Particles? Waves? Both? Neither? This video should help explain.

The folks at Veritasium have recreated Young's famous—well, amongst nerds, at least—double slit experiment. It's one of the nicest demonstrations of the phenomenon I've seen, so I won't bother trying to explain it myself. Just watch the video instead. [YouTube]