He probably wouldn't have done it if his dad had been home... 

Luckily they were on holiday at the time, and we got another piece! 

What did your mate's dad do to him..? 

I remember my school mate Alistair putting some in a plastic Coke bottle containing vinegar, in his dad’s shed one weekend!
He put a pea sized lump in, then screwed the lid on tightly………

……..it blew the window out of the shed! 

Yeah, it was interesting..! Got it round ok though, put all the passengers on the next bus and limped it back to the garage, fortunately it was only a couple of miles away.

What, not even any kids..? 

Fortunately there was no wildlife in the swimming pool... 

I've got a lovely big chunk of Na that's sitting happily in a container of oil but I am planning on demonstrating the reaction to my daughter later this year by dropping a piece of it into the river from a bridge one evening when the river is fast flowing. I don't feel comfortable making NaOH when the river is not fast flowing as I have no intention of harming any wildlife.

https://cen.acs.org/articles/93/web/2015/01/Sodium-Potassium-Really-Explode-Water.html

A guy on youtube recently researched why sodium actually explodes on contact with water. It is sure 150+ years old school demo reaction, but no one knew what is going on in the exact details.  It turned out that there are some wicked physics behind this, and not, it is not hydrogen that explodes. Actually, if sodium reacts with the water  as a chunk, is shall not explode as contact area is rather low. He found that after sodium melts on a slow reaction, it suddenly forms multiple spikes on the sodium-water interface, increasing contact area multiple orders of 10's. And then reaction goes much faster explosion-like. It is not well understood why the spikes are formed, probably because of electric field (!) generated by Na-H2O reaction itself.