Oceans · ages 6 to 12

Why is the ocean salty?

The salt comes from rocks on land, and it has been piling up in the sea for a very, very long time.

Mountains, a river and the ocean, showing how rain and rivers carry salt from rocks to the sea
Illustration from 100,000 Whys for Curious Kids.
The short answer: rain slowly dissolves tiny bits of salt from rocks, rivers carry it to the sea, and when seawater evaporates, only the water leaves. The salt stays behind and builds up.

How it works, in 3 steps

1. Rainwater is slightly acidic, so over time it wears away rocks and dissolves minerals.
2. Streams and rivers carry those salts to the ocean. Hot vents on the seafloor add more.
3. The Sun evaporates ocean water, but the salt can’t evaporate. Over millions of years it built up until about 3.5% of seawater’s weight is salt.

Myth or fact?

“River water has no salt at all.” Myth. Rivers carry tiny amounts of dissolved salt, far too little to taste.

“Salt leaves the ocean when water evaporates.” Myth. Only the water evaporates. The salt stays in the sea.

“Every part of the ocean is equally salty.” Myth. Saltiness varies from place to place.

Quick fact: if you spread all the salt in the ocean over the land, it would make a layer more than 500 feet thick.

Try it: make your own sea salt
  1. Stir a spoonful of salt into a little warm water until it disappears.
  2. Pour it onto a dark plate and leave it in a sunny window for a few days.
  3. The water evaporates. What’s left on the plate?

100 big questions like this one

This question comes from 100,000 Whys for Curious Kids by Daniel Why: a full-color visual encyclopedia for ages 6 to 12, with a labeled diagram for every big question, 100 Quick Whys and 300 myths busted. Out October 13, 2026.

Join the launch teamFree Myth or Fact cards

Sources: NOAA National Ocean Service, “Why is the ocean salty?”; USGS Water Science School, “Why is the ocean salty?”.