Why do footprints stay on the Moon for so long?

Have you ever pressed your bare foot into soft sand? Feel how it sinks down a little. When you lift your foot, a footprint stays behind. You can even see each little toe.

Far up in the night sky, the Moon has footprints too. They have stayed there for a very long time. Why do footprints stay on the Moon for so long?

Long ago, astronauts flew to the Moon and walked on it. They wore big, thick boots to keep their feet safe. Step by step, they walked across the grey ground.

The Moon is covered in soft grey dust, like fine flour. Each astronaut pressed a boot into that dust with every step. Every step left a clear footprint.

Here on Earth, footprints in sand do not last long. Wind is air that moves, and it pushes the sand into the holes. Rain smooths them away too. Soon the sand looks smooth and flat again.

But the Moon has no air at all. The Moon is airless, which means having no air around it. Without any air, what do you think happens to the wind there?

There is no wind on the Moon, not even a tiny breeze. An airless place has no rain clouds either. Not one raindrop ever falls there. The Moon stays still and quiet, day after day.

So nothing comes along to wipe the footprints away. The dust stays right where it is. Each footprint keeps its shape, just as the astronaut left it, for a very long time.

Let's remember our word: airless. The Moon is airless, so no wind or rain comes to wipe the footprints away.

Sweet dreams, little one, of soft grey footprints resting on the quiet Moon.