Sodium is an extremely reactive metal and the reaction with water produces jets of hydrogen gas below the waterline. When the sodium gets trapped under the piece of wood, enough heat can be generated to boil the water around the metal and actually melt the sodium. If the sodium gets hot enough, the hydrogen jets can ignite and burn. Burning hydrogen makes the temperature rise even faster. The rate of the reaction increases so quickly that an explosion may occur! Molten sodium can be thrown out of the dish and leave burns on skin. Wear goggles and stand behind a safety shield when you're anywhere near this reaction.