A liquid presses, and the height of the column is what counts

You've seen water tanks on rooftops. Why are they always up high? First: do liquids press at all?

Try it (Activity 6.1). Take two clear pipes the same length (about 25 cm) but different widths, one broad and one narrow. Tie a good rubber balloon over one end of each and clamp them upright on a stand. Fill both with water to the same level, about halfway, and look at the balloons.

They bulge exactly the same amount. That's surprising: the broad pipe holds much more water, so a lot more weight. If weight made the balloon bulge, the broad pipe's balloon would bulge more. It doesn't.

So it isn't the weight of the water. It is the pressure of the water column, and equal heights give equal pressure, however wide the pipe. A tall thin glass of water presses harder on its bottom than a wide pot filled only a little way up.

Now pour more water into one pipe, then a little more each time. Its balloon bulges more and more. The pressure a liquid makes depends on the height of its column.

That's why overhead tanks are put up high: the taller the column of water above the taps, the greater the pressure, and the better the water flows.