Sodium is an extremely reactive metal. The moment it comes into contact with water, it reacts rapidly, producing hydrogen gas and a large amount of heat. The heat can ignite the hydrogen, creating flames, loud bangs, or even an explosion depending on the amount of sodium and the conditions. With a large piece-such as half a kilogram-the reaction becomes dramatically more intense, sending water and steam high into the air in a spectacular display of chemistry. Although fascinating to watch, this is a dangerous chemical reaction that should only be carried out by trained professionals under controlled conditions with proper safety precautions. It's one of the most dramatic examples of how reactive alkali metals can be when they meet water. - @howitworkssss