Adding sodium to water is a stable of high-school chemistry classrooms. If you've seen it before, it's usually done with tiny nuggets of sodium and a puny water bath, for fairly sensible reasons. Well, this video by EatsTooMuchJam shows what happens when you sling a frisbee made out of 0.45 kilograms (1 pound) of sodium into a river.
As you can imagine, this isn’t one you should do yourself, kids. That violent chemical reaction is releasing a fair amount of hydrogen gas and sodium hydroxide, which isn’t going to do that river or its inhabitants any good.