We had to take a deep breath to carry out our practical investigation into lung capacity....

1. Take a deep breath and blow  into a bottle that is full of water (submerged upside down in water) - displacing water and replacing with air.

2. After completely exhaling, use tongue to form a seal over pipe opening while level of displaced water is marked on the bottle.

3. Lung capacity should equal the amount of water displaced - measure the amount of water contained in the bottle up to the marked level.