When we walk along an even field it seems to be flat.
The sea also seems flat. A long time ago that is what people thought. They thought that the Earth was flat and the sky was like a cap on top of it.
Now everyone knows that the Earth is round. But how can test that?
When a ship goes out to sea it does not disappear all at once. First the hull sinks behind the horizon. Then the deck moves out of sight. And then the mast, which you can still see for a long time, finally disappears.
That is because the surface is curved. And it is curved because the Earth is round.
