Why is the sea salty? Ocean questions kids ask, with simple answers

By Verity's mum · 23 September 2026

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Preschoolers ask brilliant questions. Usually at bedtime. Usually ones you can't answer. Here's a cheat sheet: a simple answer you can say to a four year old, plus a bit more for you. Each one is checked against a proper source, linked underneath.

Why is the sea salty?

Say: "Rain washes tiny bits of rock into rivers, and the rivers carry them to the sea. Some of those bits are salt."

For you: Rocks on land are the main source of salt in the sea. Rain is slightly acidic, so it wears rocks down and releases minerals that rivers carry to the ocean. Openings in the seafloor add some too. Source: NOAA

Why is the sea blue?

Say: "Sunlight has all the colours in it. Water soaks up the red bits, so the blue bits are left for us to see."

For you: Water absorbs colours at the red end of the light spectrum, which leaves blue. The sea can look green or brown when light bounces off sand and tiny particles in the water. Source: NOAA

What makes waves?

Say: "Mostly the wind. When the wind blows across the sea, it pushes the water into waves."

For you: Waves are energy moving through water. Most of the waves you see at the beach are made by wind blowing across the surface. Source: NOAA

Why does the sea go in and out?

Say: "The moon pulls on the sea, a bit like a magnet. That makes the water come in and go out every day. It's called the tide."

For you: Tides are caused by the gravitational pull of the moon and the sun. (It's gravity, not magnetism, but the magnet idea works for little ones.) This is why rock pools appear at low tide. Source: NOAA. Ready to try it? See our rock pooling guide.

How deep is the sea?

Say: "In the deepest place, it's deeper than the tallest mountain is tall."

For you: The deepest known point is Challenger Deep in the Mariana Trench in the Pacific, about 10,935 metres down. The average depth of the ocean is about 3,682 metres. Source: NOAA Ocean Exploration

Can turtles breathe underwater?

Say: "No! Sea turtles breathe air like us. They swim up to the top to breathe, and they're really good at holding their breath."

For you: Sea turtles are reptiles. They need air to breathe and come onto land to lay eggs, but spend most of their lives underwater. Source: Smithsonian Ocean

Is a starfish a fish?

Say: "Nope! That's why scientists call them sea stars. They don't have fins or gills like fish."

For you: Sea stars have no gills, scales or fins. There are about 2,000 species, and they all live in the sea. Source: NOAA

How many hearts does an octopus have?

Say: "Three! And its blood is blue."

For you: Two hearts pump blood through the gills and the third pumps it round the body. Their blood is blue because it uses copper to carry oxygen. Source: Smithsonian Ocean

When you don't know the answer

Say "I don't know, let's find out." Then look it up together in a fact book. That's a better lesson than any fact. Our ocean picture books guide includes a first fact book with big real photos, made for exactly this.