Why is seawater salty?
The short answer
Rain and rivers dissolve small amounts of minerals from rocks and carry them to the ocean. Water evaporates, but most salts remain.
How it works
Seawater contains ions such as sodium and chloride. Undersea volcanic activity and reactions with seafloor rocks also add dissolved material. When water evaporates into clouds, most of the salt stays behind.
A familiar example
If seawater dries in a shallow container, crystals remain because the water leaves while dissolved salts stay.