But a flat earth would look and behave completely differently to what we actually see.
Being able to sometimes see slightly further than expected is due to refraction. Refraction is well understood, except by the person narrating this video, who just tries to dismiss it by ridicule. Refraction has the biggest effect near water.
(from
Seeing beyond the Horizon)
While a flat earth is far simpler to understand than a rotating globe earth, it does not agree with everyday observations.
There is an abundance of evidence that the earth is a globe, much of which contradicts a flat earth explanation. Here are some examples. Perhaps start by explaining how sunrise and sunset works on a flat earth.
Sunrise
Sunset
Remember that any explanation of sunrise must also explain sunset, midday sun and no sun at night, all at the same time for different places on the (allegedly) flat earth.
Size of the sun does not significantly change and always looks round.
You can see further out to sea from the top of a cliff or hill than lower down. For example, you cannot see an island 20 miles way when at or near sea level, but can see if from the top of a hill.
Ships go out of sight over the horizon from the bottom up.
Intercontinental flight times.
On a flat earth, the horizon should always be at eye level, even when on a mountain. It isn't.
Phases of the moon.
Eclipses
Seasons
Does not resort to any conspiracies for support.