He can recite the poem by heart.

There is not a single cloud in the sky.

She was moved when she caught the first sight of the beautiful scenery.

When starting a big project, one should remember the proverb 'start with what's near at hand'.

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.