Do a working system of lungsĀ 
take a bottle cut the end of the bottle tie a ballon at the end of the bottle and inside the bottle take a straw fix it in the cap of the bottle in the straw fix another 2 straw and at the end tie a ballon in straw and close the cap of the bottle.remember the ballon at the bottom should be bigger than inside it .
after wards if u pull the ballon at the end the inside 2 ballon will expand thus from this experiment we can learn that as diaphragam expand lungs will take oxygen as it contract it gives co2