On many flights, they would give away leveling devices to all passengers for free, so that everyone would be able to observe the dip of the horizon and proves Earth is a sphere. And as a nice side effect, to quench our thirsts, too!
Such devices are called ‘bottled water’. Using these simple ‘instruments’ on a flight, we can demonstrate that Earth is a sphere.
By drinking half of the water in a bottle of water, we can use the bottle as a simple leveling device. Firmly close the bottle, hold the bottle horizontally, and aim for the horizon line, all while keeping our eyes level with the surface of water inside the bottle. If weather condition allows it, it is not hard to observe that the horizon is below the eye level.
The phenomenon is called the dip of the horizon, and can only occur with such magnitude if Earth is a sphere.
Flat-Earthers repeatedly assert that “horizon always rises to eye level, no matter how high we are from the surface.” Using these simple ‘instruments,’ it is trivial to prove them wrong.