Cut off the bottom of the bottle, and smooth the rough edges using sandpaper (or cover the rough edge with tape). Cut a large piece from one of the balloons and use it to cover the bottom end of the bottle, securing it tightly with an elastic band. The seal must be airtight.
Push the second one into the bottle, making sure you keep hold of the neck.
Carefully attach the balloon by folding the rim of the neck over over the outside of the bottle opening. The balloon should then hang inside the bottle, attached to the top. Seal the join with an elastic band.
(Alternatively, attach the balloon to a tube. Make a small hole in the bottle top, and push the tube through it. Seal around the edge of the hole with sticky putty. Hang the balloon inside the bottle, and put the lid on.
Make sure the whole system is airtight, adding tape to seal any gaps.) You now have a "lung" (the balloon), with a "windpipe" (the neck of the bottle), and "diaphragm" (the piece of balloon attached to the bottom of the bottle). You can make the lung work by moving the diaphragm gently up and down - stick a tab of tape on the diaphragm to help pull it down. This mimics the human diaphragm contracting, which increases volume in the chest cavity.
As the diaphragm contracts and moves down and creates a greater volume in the chest, the air pressure inside the chest is reduced. The air pressure outside the body is then higher so air from outside will enter the lung.
When the diaphragm relaxes (when we let go of it in our model), the volume in the lungs decreases, and air is exhaled.
Sophie Duncan is programme manager for science at the BBC www.bbc.co.ukscience