They do it at school in chemistry. We get little petri dishes of water and put potassium in. (It doesn't make a big bang, but the potassium 'explodes' and sets itself on fire, whizzing round the dish. )
I know, I remember. I'm just telling you 'bout where I've done it. Our teacher tells us stories about how as a boy, he'd throw chunks of sodium and potassium into his school's swimming pool during winter, when it wasn't used.