When you put the pen top in the water, a bubble of air
is trapped inside it. This trapped air bubble makes the diver
less dense than water, so it floats. When you squeeze the bottle,
water squashes the air and takes up more space in the pen top. Now
the diver is denser than water, so it sinks. When you relax
your grip on the bottle, the air in the pen top expands again. Now
the diver is less dense than water, so it comes back up to the surface.
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