Lung capacity — the total air your lungs can hold — varies from person to person, and a balloon makes the difference visible. You give identical balloons to several people and have each one blow up their balloon with a single breath. Wrap a tape measure around it, record the size, and repeat two more times to get an average. Comparing results across people shows whether body size or fitness level affects how much air the lungs hold.

Lung capacity — the total air your lungs can hold in one deep breath — varies between people with different habits, and between boys and girls. You recruit 20 participants of the same age: half are choir members with at least one year of experience, the other half are non-singers and non-athletes. Each person puts on a nose clip, takes one deep breath, and blows as much air as possible into a balloon. You then measure the balloon diameter with a ruler. Choir members inflate their balloons to a larger size than non-singers, and the results also show that boys tend to have greater lung capacity than girls.

Lung capacity is one way to measure fitness, and a water displacement method makes the measurement visible. You fill a bottle with water, turn it upside down, and have a volunteer blow through a tube into it. The air pushes water out, and the markings on the bottle show the volume of air from one deep breath. Testing volunteers with different fitness levels reveals whether more active people hold more air.