Simple experiment demonstrating how much air lungs can hold
Plastic 2litre pop bottle
Bendy straws (one each)
Fill the pop bottle and bowl with water (full to the brim).
Pop the short end of the straw just in to the nectk of the bottle
Quickly, but carefull, turn the pop bottle upside down so the neck of the bottle is under the water in the bowl
Get the BS to take a good breath and blow through the straw in one continuous breath - as they breathe out, the water in the bottle displaces, giving an indication of how much air they have breathed out (i.e. the missing amount of water in the bottle).