It's a bit comical that in 2017, some people are still debating whether the Earth is round or flat. In fact, the International Flat Earth Society, founded in 1956, is still up and running today.
According to the Flat Earth Society wiki page, the biggest evidence for their belief comes from "relying on one's own senses to discern the true nature of the world around us." Moreover, they write: "The world looks flat, the bottoms of clouds are flat, the movement of the sun; these are all examples of your senses telling you that we do not live on a spherical heliocentric world."
But of course many scientific observations have disproved and discredited the flat Earth hypothesis. If the many images of Earth taken from space still aren't enough to convince a Flat-Earther, here is a simple way to prove that the Earth is, indeed, round. Try placing two sticks at different distances in the ground and observe the sticks' shadows. If the Earth is flat, the shadows should be identical regardless of the sticks' distance. However, because the Earth is curved and the sun hits the sticks at different angles, sticks placed at different locations will cast shadows of different lengths.
For more simple evidence-based arguments against a flat Earth, watch the video!