Distillation
Distillation is a way to clean water by boiling it into steam and then cooling the steam back into liquid.
Think of it this way
When you boil a pot of water, the steam rises up and leaves the dirt and salt behind. That steam is pure water vapor. When it hits a cold surface, like a lid, it turns back into water drops. Those drops are cleaner than what you started with.
Explaining distillation by grade level
When you heat dirty water, it turns into steam. The steam rises and leaves the dirt behind. If you cool that steam, it turns back into water. This new water is much cleaner than before.