He can recite the poem by heart.

There is not a single cloud in the sky.

When I hear that song, old memories come to mind.

When I heard his name, his smile immediately came to mind.

The air in this room is fresh.

What will you use this empty box for?

Looking at the bird flying high in the sky, I felt freedom.

The altitude of this airplane is 10,000 meters.

In mathematics, an empty set refers to a set that does not contain any elements.

I enjoyed a picnic under the clear sky.