Osmosis

The net diffusion of water molecules across a semi-permeable plasma membrane

Imagine a tea bag, the bags walls are porous (holes in them) just like a membrane.

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

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