Your lungs as swim bladders
Your own lungs still can act a little like swim bladders. Here’s a lungs project: When you go swimming at the pool or in the ocean or a lake, try taking a deep breath and floating while you hold your breath. Then try blowing out all the air, and floating.
Respiration and evolution
Evolution of fish
Fish bone project
Fish egg project
All our biology articles
For further thought
Which is easier? Do you float higher in the water with your lungs full of air, or empty? Does it matter how long you hold your breath?
History of swimming

President Obama swimming with his daughter
Find out more about breathing
History of Swimming
Bibliography and further reading for your lungs project: