Steam Pressure
Steam pressure is the pushing force that builds up when water boils in a closed space.
Think of it this way
A sealed pot sits on a hot stove. Water inside boils and turns to steam. The steam fills every corner of the pot, pushing outward on the lid and walls. The more heat you add, the harder the steam pushes.
Explaining steam pressure by grade level
When water boils, it turns into steam. Steam takes up much more room than water. In a closed pot, the steam has no place to go. It pushes hard on the walls and the lid.