This experiment was about learning about pressure in the ocean.
Steping on the bottle simulates what happens to your lungs when you dive down in the depths of the ocean. Your lungs get smaller when you dive deeper into the ocean and the pressure on your body increases. When you release the pressure from the bottle this is what happens to your lungs when you surface back to the top of the ocean. Now you can see why divers use oxygen tanks to breathe under the water. They also use them to increase the pressure in a diver's lungs so they won't collaspe as he dives deeper into the ocean.
Which is heavier: hot or cold water?