Why sodium potassium and lithium are stored under oil?

These metals are very, very reactive. This is the big reason. They react very fast with things in the air. Like the oxygen and the water vapor.

If they touch water, it is a big reaction. It can make fire or a small explosion. It is very dangerous. For sodium and potassium, this reaction is very strong.

So, we must protect them from the air. We put them in oil, like paraffin oil. The oil is like a blanket. It stops the air and water from touching the metal. The metal is safe inside. It does not react with oil.