Rain is fresh and rivers are fresh — so why is the ocean salty?

Every river on Earth pours fresh water into the sea. Rain falls fresh enough to drink. Yet the ocean they all feed is too salty to swallow. Where does all that salt come from — and why does it stay? Let's run the sun and find out.

1Two things to notice first

Water goes in a loop — and rivers aren't perfectly pure

You only need two ideas. Watch each one:

Water travels in a loop

The sun lifts water off the sea as invisible vapor. It gathers into clouds, rains back down, and runs to the sea again — round and round, forever.

Rivers carry tiny specks of salt

River water looks clean, but it isn't pure. Rolling over rocks and soil, it picks up tiny, tasteless specks of dissolved salt and carries them to the sea.

2The big question about the sun

When the sun lifts water off the sea, what goes up?

Here's the thing nobody can see. The sun's heat lifts water off the sea every day — but does it carry the salt up too, or leave it behind? There are two ideas:

Idea A

Water and salt rise together

The sun scoops up the whole salty mix, so the vapor — and the rain it becomes — is a little bit salty too.

Idea B

Only the water rises

The sun lifts pure water and leaves every speck of salt behind in the sea, so the vapor and rain are fresh.

3Your turn — run the water cycle

Move the sun and watch the loop spin

Drag the slider to make the sun hotter or cooler. Watch water rise off the sea, become a cloud, rain down, and flow back through the river — the never-ending loop. (We'll worry about the salt next.)

☀️ Sun's heat warm
COOL · SLOWHOT · FAST
sun's heat water

4Now test the two ideas

Run a thousand years and see what the salt does 🌊

Same sea, same rivers trickling in salt. The big test: pick what the sun lifts, then fast-forward the years and watch the saltiness meter. But guess first — then run it.

Guess before you run the years

When the sun evaporates seawater, does the salt rise up with the water — or stay behind in the sea?