The best argument against a round Earth is, why hasn't everything south of the equator fallen off?
But if you look at a map you'll notice there's a lot less land than in the northern hemisphere, so maybe a lot of it did fall off. Only sticky land remains, like Antarctica which hangs from the bottom like an icicle.
That doesn't explain how southern-hemisphere people stay on the ground. But that's because gravity is only half the equation. Earth has a powerful magnetic field which keeps you stuck to the ground even if gravity fails, as long as you eat enough metal. Southern hemisphere lands are full of metals, for example Argentina, South Africa and the Gold Coast of Australia.
Because this magnetic force is much weaker, it only works if you are very near the ground. And that is why nobody flies over Antarctica. You may have heard rumors about an "ozone hole" there, well in fact there's hardly any air at all, because it has all fallen off. Only specially-designed planes can operate there.
So you see the Earth is indeed round.