When water is in the gas form, we call it water vapor. In this state, the particles of water are much farther apart compared to when it's a liquid or solid. They are so far apart that they can move around freely.
The particles in gas move really fast, bouncing off each other and spreading out. When they collide, it’s a quick bump, and they zoom away in different directions, like little balls on a pool table. This fast movement and spacing is what makes gases fill up the space they're in!