Try this: Balloon lungs in a bottle

Safety: This activity involves cutting a soft drink bottle. Younger readers should get an adult's help.

You will need

• A clear plastic one litre drink bottle without a lid
• Scissors
• A three way irrigation connector (about 50 cents in a hardware store irrigation/sprinklers section).
• Around 15cm of plastic tubing that will fit tightly on your connector
• Three good sized balloons
• Two rubber bands
• A large clump of modelling clay, plasticine or play doh.

What to do

1. Start with the three way irrigation connector. If you can't get one, a three way hose connector (more expensive), making your own from pipe and lots of tape or a one-lung version (see note below) are other options. Push the plastic tube onto one arm of the connector. This tube represents your trachea, or windpipe.
2. Attach the two balloons to the other two arms of the connector. Secure each of these balloons on with rubber bands. These are your balloon 'lungs'.
3. Use the scissors to carefully cut the bottom off the plastic bottle.
4. Place the balloons and hose connector inside the bottle with the tube sticking out the bottle neck. With the modelling clay, make an airtight seal between the top of the bottle and the tubing. Achieving an airtight seal is important, so add extra sticky tape to ensure that air can only enter the bottle through the tubing.
5. Now stretch a balloon over the bottom of the bottle. To do this, tie a knot in the neck of the third balloon and carefully cut the bottom half off. Stretch the balloon rubber across the bottle as tight as you can and secure well with tape. The balloon needs to make an airtight seal on the bottle. This balloon is your 'diaphragm'.
6. Now your bottled lungs are ready to take a breath. Grab onto the balloon diaphragm at the base of the bottle and gently pull it downwards. Watch closely what happens to the two balloon 'lungs' inside the bottle. Let the balloon go and watch again what happens in the balloon 'lungs'.

Note: This activity can be made with just one balloon 'lung'. In which case you do not need the hose connector and you can attach the balloon directly onto the clear plastic tubing.

What's happening?

Watch your chest as you take a deep breath. Place your hands on your abdomen just below your ribs (your diaphragm) and feel what happens when you take another deep breath. What did you feel? How does your body breathe?

When your diaphragm moves down, you breathe in. When your diaphragm moves up, you breathe out. Try taking a really deep breath. Can you feel a stretching feeling in your stomach? That's your diaphragm moving down as you breathe in.

In the model you have just made, the balloon diaphragm is pulled down and this lowers the air pressure inside the bottle by increasing the space inside the bottle. The outside air then enters through the tube, which makes the two balloons inside the bottle 'chest' expand. Your lungs work in exactly the same way.

When you breathe in, the contraction of the diaphragm increases the space in the chest, which results in a lower pressure in the lungs and a subsequent inhalation of air. When the diaphragm relaxes, the natural elasticity of the lung tissue and chest will reduce their size and you breathe out.

Your diaphragm is a muscle sheet that separates your chest cavity, where your heart and lungs are, from your abdomen, where you stomach and intestines are. The diaphragm is shaped like a dome and is found within your chest, next to and between the lower part of the sternum (breastbone), the lower six ribs, and the lumbar (loin) vertebrae of the spine.

If your diaphragm gets stuck, breathing can become difficult. This is called "being winded" or "having the wind knocked out of you". The diaphragm is not just important for breathing however, it is also important in expulsive actions - coughing, sneezing, vomiting, crying, pooing, and peeing.

Applications

Hiccups are spasmodic contractions of the diaphragm. This spasmodic movement causes a sudden intake of breath that is then involuntarily cut off by a closure of the glottis (the opening between the vocal cords). This produces the characteristic sound of a hiccup.

Hiccups remain a source of mystery; there is no one clear reason why we get them, or how to get rid of them. Hiccups can arise from various causes, most common being an over stretching of the stomach. Gastric irritation, nerve spasms, and various metabolic disturbances may also cause hiccups. They usually stop within minutes, although they may persist for days or weeks, and there have been isolated reports of hiccups continuing for several years.