It's not round bottom. It's flat. Well, slightly concave. Check this out, this time it's an art term called "perspective", take that same ball jar from our last exercise and tilt it toward you just a little bit. Ever see a picture of a guy pinching the moon? It's the same sort of thing, albeit a little bit more dramatic. He's not really pinching the moon, it's really far away and super big, that would be impossible. It just "looks like" he is because he's close and the moon is really far away. What you're really seeing is the curve where the squarish sides become the circular bottom, the flat part is being obscured by the rest of the jar and the opaque liquid inside of it.