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.