The answer is the lid of a bath tub.

The picture’s lid is the most popular type which is made of plastic.

Every family uses it. We use the lid to prevent warm water from getting cold.

Why do we keep water warm?

Almost all Japanese families take a bath at night.

First we fill a bath tub with warm water.

When it is full, we cover it with the lid, and then take a bath in turn.

In other countries, some people wash their body in the tub.

But in Japan, people wash their body out of the tub.

The bath tub is used to warm our body.

It is a custom for us from long time ago.

If we don’t warm in the bath, we don’t feel taking a bath.

I think the reason is relaxation.

Having a bath leisurely and sweating is said to be good for our health.

And it’s also good for saving the charges for gas.

Keeping water warm and taking a bath in turn saves us extra energy to reheat water.

Don’t you think it’s reasonable to use the lid?

That’s all why we use it.