Column A: Let's assume we have a column of water of height 2m. And also assume water density is 1kg/m^3. Then the pressure at the depth of 1m is pgh = 1*g*1 = g, which means the top 1m of water is pushing down with pressure g, and the water right past 1m mark is pushing up with pressure g. Also, at the end of the 2m, the pressure is pgh = 1*g*2 = 2g. Column B: Ok, and then say we remove the top 1m of water and replace it with oil of density 1.01 kg/m^3. Now, the oil is supposed to sink to the bottom of the whole column because it's denser, so it should eventually become 1m of oil at the bottom. But let's assume for the sake of argument that the oil doesn't sink, and so the pressure at 1m is pgh = 1.01*g*1 = 1.01g. At height 1m, the oil in Column B pushes down with pressure 1.01g. In real life, this wouldn't happen, but the question is why, in real life, doesn't the water right past the 1m mark also push up with pressure 1.01g? We've shown that water is capable of pushing up to 2g of pressure at least, so why doesn't the water right below the 1m mark just push up 1.01g? Why does water care about the density of whatever is on top of it? Or another way to ask is: The water at height 2m has pressure 2g. The pressure at 1m is 1g. But is the water at 2g physically any different than the water at 1g? If both waters are the same material, and if one water is capable of sustaining 2g pressure, then why would the other water not be able to also sustain 2g of pressure?