Air pressure
Air has weight, so it presses on the ground and on everything on it – this is atmospheric (air) pressure. We do not feel it because the pressure inside our bodies balances it. In 1643 the Italian Evangelista Torricelli measured it with a mercury barometer. At sea level and 0 °C normal atmospheric pressure equals 760 mm of mercury (about 101 kPa). As height increases the air gets thinner so the pressure falls: for each 10 m rise above sea level it drops by about 1 mm of mercury (a good estimate only at fairly low heights; much higher up the fall slows down). That is why breathing is harder in the mountains and ears can “pop”. Pressure also changes with weather: a quick fall often means rain or a storm is coming. Today safe aneroid or digital barometers are most often used.
Experiment: fill a clear plastic bottle half with water and invert it with its mouth under water in a basin, watching the water level; then fill a plastic cup with water, lay a stiff card on top, hold it with the palm and carefully turn it over – the card stays because of air pressure. Safety: work over a basin or tray, wipe up spills at once (slipping risk) and use a plastic cup instead of glass. Never break a mercury thermometer or barometer; mercury is poisonous.