I still think tho that earth is round, well maybe not perfectly round but ellipse.

Yet, I know some people think (even my brother and sister in law) that earth is flat.

Good idea.

Somehow, ya know those flat earth people's mind is locked within stubbornness.

Whatever they said that I am still keeping my mind on to ancient knowledge, fine.

You know some engineering disciplines still use outdated physics which, whilst academically incorrect math for calculating things like orbiting satellites, still works just fine for things like constructing an electrical engine.

For a person who is going to spend their life using pedal power and livestock, for example the shape of the Earth, being so big really makes no difference, since all the physics related to their experience and needs work just as if the Earth was flat.

Since such people are generally religious pentacostals and religion is intentionally allegorical, you can just safely assume they're being allegorical when they say the Earth is flat, even if they don't know it. Just pretend they're saying "might as well be as far as my life goes".

The main thing is that they're happy and you're happy. If it ever comes to something important where the distinction must be made, you have physical evidence on the side of scientific method and emotional anxiety on the side of wonton fantasies as a form of argument. However it is more important as social beings to avoid emotional arguments with other people simply for the sake of it and sometimes the courage to do the right thing means keeping your trap shut.

In psychotherapy a legitimate contributor to irrational behaviour is a history of abuse, in laymans terms it means a person who has irrational conclusions can very easily turn surprisingly dangerous to someone that isn't expecting it. Childish thinking in an adult is a danger sign, not a comedy act. Unless physically confident, let them think any silly thing they feel like if it keeps them calm.

The flat Earth model is an archaic conception of Earth's shape as a plane or disk. Many ancient cultures subscribed to a flat Earth cosmography, including Greece until the classical period, the Bronze Age and Iron Age civilizations of the Near East until the Hellenistic period, India until the Gupta period (early centuries AD), and China until the 17th century.

Historian Jeffrey Burton Russell says the flat-Earth error flourished most between 1870 and 1920, and had to do with the ideological setting created by struggles over biological evolution. Russell claims "with extraordinary few exceptions no educated person in the history of Western Civilization from the third century B.C. onward believed that the Earth was flat", and ascribes popularization of the flat-Earth myth to histories by John William Draper, Andrew Dickson White, and Washington Irving.

Modern flat Earth societies consist of individuals who promote the idea that the Earth is flat rather than a sphere. Such groups date from the middle of the 20th century; some adherents are serious and some are not. Those who are serious are often motivated by pseudoscience or religious literalism.