...but wait a minute. You "know" it's round because why?
That was not addressed to me, but I'll respond.
In the beginning, it's because what I was taught in school and by my elders. I was also taught Santa Clause existed and, later, Santa Clause does not exist.
Beyond that, I never really gave it much thought, until I came upon people, mostly on line, who believed otherwise. So, there is some controversy. If there is controversy, then one must find evidence - pro and con.
For starters, one can use pictures of earth from ISS, from the moon, from satellites. Then one must determine if the evidence is valid.
Here we have two options; they are real or they are artificially manufactured. If they are artificially manufactured, one would have to question WHY and WHO. Who would gain from telling everyone the earth is a sphere when it's really a Frisbee.
From comments on boards like this, there are no rational answers as to WHY and WHO. From further research, the WHY and WHO are a small community of people who make money selling books, maps, and tee shirts and make more money by holding and speaking at conferences.
Alternatively, if they are real, one can also ask WHY and WHO. The WHO supporting a spherical earth are tens of thousands of scientists, explorers, sailors, pilots, etc. over many hundreds of years who have no agenda other than recording their observations for the benefit of mankind.
Beyond just pictures, mankind has made a model for a spherical earth orbiting a distant sun that is in accord with observable nature.
FE proponents do not have such a model. There is no single model that addresses day time, night time, seasons, moon phases, eclipses, tides and the "movement" of the stars and planets across the sky. FEs' cannot explain Mars's apparent retrograde. FEers cannot even define basics like the distances to the sun and moon.