Here are some ways to prove to a flat-Earther that the Earth is round on the spot, without any studies or preparations:

• Watch a ship sail over the horizon. If the Earth were flat, you would be able to see the ship get smaller and smaller as it sailed away, but it would never disappear over the horizon. However, if the Earth is round, the ship will eventually disappear over the horizon as it sails away.
  

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• Quora

• Ship sailing over the horizon

• Look at the stars. If the Earth were flat, the stars would always appear in the same place in the sky. However, if the Earth is round, the stars will appear to move as you move around the Earth. This is because the Earth is rotating on its axis.
  

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• Forbes

• Stars moving in the sky

• Take a flight. If you take a flight, you will see that the Earth curves away from you as you ascend. This is because the Earth is round.
  

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• Financial Times

• Earth curving away from a plane

• Look at a globe. A globe is a model of the Earth that is round. If you look at a globe, you will see that the Earth is round.
  

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• Earth 3D Map

• Globe of Earth

• Talk to a scientist. Scientists have studied the Earth for centuries and have concluded that it is round. If you talk to a scientist, they will be able to tell you more about why the Earth is round.

It is important to note that some flat-Earthers may not be convinced by any of these proofs. They may believe that the Earth is flat, no matter what evidence you show them. However, if you can get a flat-Earther to observe the Earth for themselves, they may be more likely to believe that the Earth is round.

It is also important to be respectful of flat-Earthers, even if you disagree with them. They are entitled to their own beliefs, and you should not try to force your beliefs on them. If you can have a respectful conversation with a flat-Earther, you may be able to help them understand why the Earth is round.