Nucleation
Nucleation is when tiny bits of dust or smoke give water drops or crystals a surface to form on.
Think of it this way
Bubbles form on the rough bottom of a pot before they appear anywhere else. The scratches and bumps give bubbles a spot to start. Smooth glass has fewer spots, so fewer bubbles form. Dust and smoke work the same way for water drops in the air.
Explaining nucleation by grade level
When you breathe out on a cold day, your breath makes a small cloud. That cloud needs tiny bits of dust to start forming. Each speck of dust helps water drops grab on and grow. Without dust, the cloud would not appear at all.