ELI5: How does osmosis work?

Osmosis is like water sneaking through a tiny doorway to make everything have the same amount of "stuff" dissolved in it.

Imagine you have two rooms separated by a wall with a tiny, tiny door that only water can fit through. In one room, you have a lot of sugar dissolved in the water, making it super sweet. In the other room, you have just plain water.

• The water wants to even things out, like sharing!
• Because there's more water without sugar in the plain water room, the water molecules start to go through the tiny door into the sugary room.
• They're trying to make the sugar concentration (how much sugar is in the water) the same in both rooms.

Think of a raisin in water. A raisin is like the sugary room. If you put a raisin in a glass of water, the water outside the raisin has less "stuff" dissolved in it than the inside of the raisin. So, water will move into the raisin through osmosis, making the raisin swell up! The raisin's skin is the semi-permeable membrane.