All you have to do is get out on a day with good viewing conditions and take a look around. There is no horizon dropping as should be on a spherical object (for comparison check Google Earth). And what's more, the higher you get, the further you can see. Again, still no horizon dropping.

If you think this is insignificant - do the math. Take the official curvature formula (NASA, Wikipedia or online curvature calculators, for your convenience) and calculate how far you should be able to see, then compare with your experience.

Try to curve liquid water ... you will fail.

Try to prove gravity (after you have learned WHAT gravity is) and you will fail.

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