1. Cartesian Diver model ( Physics toy used to demonstrate Archimedes’ principle). 
   
   
   
   

                                                                     

           

This straw diver contains an air bubble sealed inside. Without squeezing the bottle the overall density of the diver is slightly lower than that of the water, so it floats. When you squeeze the sides of the bottle you increase the pressure on the air bubble, compressing it into a smaller space.

This decrease in volume of the bubble causes an increase in the overall density of the straw diver. When it becomes greater than that of the surrounding water it sinks. Releasing the pressure (by releasing the bottle from your grip) allows the air bubble to expand back to its normal size, and the straw diver floats again.