ELI5: osmosis

What is osmosis?

Osmosis is when water molecules move across a special barrier (called a membrane) to balance out salty or sugary stuff on both sides, like how water sneaks through a paper towel to spread out salt evenly [2]. It's the water trying to make things fair and balanced, even though the water molecules themselves don't know what they're doing [2].

Why does it happen?

Water molecules are always bouncing around randomly, but they bounce more toward the side that has more dissolved stuff (like salt or sugar) because water gets stuck to those particles, so fewer water molecules bounce back out [2]. Eventually, more water ends up on the salty side than the non-salty side [2].

What does it feel like?

Imagine you have a party on one side of a rope and no party on the other side—people keep wandering toward the party side more often than leaving it, so the party side gets more crowded over time [2]. That's what happens to water molecules around salt or sugar [2].

How does osmosis help living things?

Osmosis is how your cells drink water and stay plump and healthy—if water couldn't move in and out of your cells, they would shrivel up or explode like balloons [2].