Student #2: Dude, relax. You can kill 2 birds with 1 stone, just use osmosis.
The water displaces inside the bag, now although this has a lot to do with environmental pressure (ie. the water) it is mostly to do with diffusion, Osmosis is a form of diffusion*, specifically across a semi - permeable (porous) membrane.
Diffusion is displacement of molecules so that they are equal.
E.g. Imagine your friend sprays deodorant/perfume on the far side of the room, it takes a couple of seconds to reach you.
It is a highly concentrated cloud at first but then spreads (diffuses) into the air until there are an EQUAL concentration of spray to the air.
E.g. five 1 pennies to every three 2 pennies