Salinity is the amount of salt dissolved in water. Ocean water has high salinity because it contains large amounts of dissolved salt, which is why it tastes salty and can sting if it gets in your eyes. Fresh water from rivers and lakes usually has low salinity, which is one reason people can drink it.

Scientists measure salinity to understand water systems. The Dead Sea has such high salinity that swimmers float easily on its surface without even trying. The Great Salt Lake in Utah works the same way. Meanwhile, the Baltic Sea has lower salinity than most oceans because so many rivers flow into it, constantly adding fresh water that dilutes the salt.

Salinity matters for living things too. Saltwater fish can't survive in fresh water, and freshwater fish can't live in the ocean. Their bodies are adapted to specific salinity levels. Even slight changes in salinity can affect which plants and animals can thrive in a particular body of water.

When water evaporates from the ocean, it leaves the salt behind, which is one reason ocean salinity stays fairly constant over time. Understanding salinity helps scientists track ocean currents, predict weather patterns, and monitor the health of marine ecosystems.