system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How water gets around to the rest of my body after I drink it.
gpt
So cells all have a membrane, which acts like a border. This membrane separates what's inside the cell from what is outside. If you're looking to get into the cell, you have two ways of doing it. 

The first is called passive transport, and it involves things like water. Passive transport requires no energy on the part of the cell, so in this context it's similar to holes in the fence that let in certain things (bugs, air, etc), so the border patrol doesn't have to get up and open the fence. 


There's also an active transport system, which does take energy. There are a bunch of types of active transport, the specifics of which aren't completely necessary to know right now. This would be like how someone from the outside would have to go through customs to enter the country, or a citizen with special clearance. Sometimes, your country feels like invading another one, so they move the fence to encompass the new area. This process uses energy, as the doors to the border have to be opened, and closed, and you have to pay the staff, and sometimes you have to move the whole goddamn fence!


Now, onto osmosis. Water is a hipster molecule. It wants to go to the places where it doesn't really exist in high concentrations. So let's say you take a pot of water and separate the pot into two with a special filter that will let water through between them, but nothing else. Then let's say you pour a bunch of sugar into the first half of the pot. Remember, the filter lets water flow freely, but the sugar can't. The sugar is stuck in the first half. So now what you have is a difference in water concentration. There's a much higher concentration of water on the second side, the one that has no sugar. So water really wants to even out its concentration and flows into the side of the pot that has all these sugar molecules floating around. It does this until the concentration of water is relatively the same in both sides. This is called osmosis, and this is usually how water gets around the body. 

In the body, water flows into areas with a very low concentration until there's an even amount everywhere. But sometimes, some areas need to be free of water. This is where active transport comes in. If there's too much water in a cell, active transport is the border guard that kicks the excess water molecules out. Sometimes, when an area is really sensitive to water, the cell builds a membrane that is impermeable to water, which is like building a wall that goes 100 meters below, and above ground.

**TL;DR: Water is an illegal immigrant, and the cell has very efficient border patrol**

**Edit: Spacing, TL;DR, wrote down more stuff, etc.**