Evidence

The earth is flat and I can prove it using gravity. people believe that gravity or the force that causes masses to attract one another. If the earth was round, then shouldn't the water be completely level if it was being pulled down? Then why is the water still on the sides, its as if there was a giant mass that wasn't below, but instead surrounding the cup. Its the only reason I could think of, but this has to be conclusive proof that the earth is flat