Here's a good cooking tip: if you want to make tasty, crispy french fries from scratch, soak your potatoes in salt water. You might be wondering what that has to do with salt water fish surviving in fresh water, but it'll become clear soon.

When you soak the potatoes in salt water, the potato loses water and so when you throw your french fries in the deep fryer, they come out nice and crispy. This happens through a process that goes by the scientific term "osmosis". For now, osmosis can be thought of simply as water trying to move from an area that has less salt to an area that has more salt. Potatoes in salt water is one example of osmosis. Osmosis happens all the time in everyday life, and fish use osmosis to maintain a careful balance of salt and water in their body.

A fish that lives in freshwater will drink water, but fish have more salt in their bodies than the freshwater has. Osmosis says wants to get to an area of less salt to an area of more salt, so the freshwater will try to get into the fish. That means the fish has to get rid of the water in the same way that we do: using the bathroom! The opposite is true for saltwater fish. Saltwater has more salt in it than the amount of salt that is in a fish. So water will try to go from the fish (area of less salt) to the saltwater (area of more salt). That means a saltwater fish has to continue to drink a lot of water but doesn't have to use the bathroom as much.

So, what happens when a saltwater fish gets put into freshwater? The direction of water due to osmosis is reversed. Instead of water going from the fish to its environment, the fish begins to take in lots of water - much more water than its used to. It is drinking more water than it can get rid of, and the result is not good. Poor fish. The chemicals in the fish's body, which keep it alive and well, are not balanced correctly, and the fish will soon die.

The opposite is true for freshwater fish. In saltwater, they get too much salt and end up dehydrated. The same thing can happen to any other animal if they drink too much water or if they don't drink enough water (but don't worry, it's almost impossible to do this on accident.) But with fish, their drinking is controlled by their environment, so a saltwater fish in freshwater can be really bad.

As a side note, there are some types of aquatic life that survive in freshwater and saltwater. Salmon is a good example. But even salmon need a little time to get used to the change in saltiness. They can't just dive into the ocean straight from a freshwater stream. It's a gradual change. If you want to learn more about these kinds of animals, you can search for "euryhaline", which just means animals that like a wide range of salty environments.