Ventilation is the process of moving fresh air into a space and stale air out of it. When you open a window on a stuffy day, you're creating ventilation. The fresh breeze that flows in pushes out the old, stale air that's been sitting in your room.

Buildings need good ventilation to stay healthy and comfortable. Schools use ventilation systems to bring in outdoor air and remove the air that students have been breathing all day. Without proper ventilation, a room fills up with carbon dioxide from people's breath, gets too hot or cold, and can even trap odors or harmful particles in the air.

When something has good ventilation, air moves through it freely, like a well-designed barn with openings that let breezes flow through. Doctors also use the word ventilation to describe how your lungs bring oxygen in and push carbon dioxide out with each breath.

You might notice buildings have vents in their walls or ceilings: those metal grates are part of the ventilation system. Mines need powerful ventilation to bring fresh air deep underground where miners work. Even your computer has ventilation, with tiny fans that pull cool air in and push hot air out to keep the electronics from overheating.