2 liter bottle filled to the brim with water, add in a test tube upside down half filled with water, add cap?

Question

with the cap tightened, why does the test tube sink to the bottom when you squeeze the bottle, and float back up when you let go?

Answer 1

There is a half test tube of air in there. The tube floats because there is enough buoyancy (air-displaced water) to make it float. When you squeeze the bottle you compress the air so that it takes up less volume (meaning less buoyancy results from that lesser volume of air). Then it sinks for as long as the bottle remains squeezed.