In one sentence, describe what the following article is about:

A flat area without any trees or buildings is best, like an open field or the beach. This experiment works best on a day with clear skies. Make sure you can still see the horizon when you're laying down. If you can't, look for a spot with a clear view. You want to stand up as soon as the sun sets beneath the horizon. When you do, notice how you can see the sun setting over the horizon again. Think about it: if the earth was flat, it wouldn't make a difference if you stood up quickly after the sunset the sun would still be gone below the flat edge of the earth. But since the world is round, standing up lets you see further over the earth's curve. That's why you're able to see the sunset twice! If you were to let the sun set again, and then quickly get up to a higher viewpoint, you could see the sunset for a third time!

Summary:
Find a spot outside just before sunset where you can see the horizon. Lay down on your back and turn your head toward the sun. Wait for the sun to set and then quickly stand up to see the sun again. Ask yourself why a round earth makes this possible.