Pressure in a gas and a liquid at rest
A liquid presses on the bottom and walls of its vessel because of its weight. This is hydrostatic pressure and is found with p = ρ · g · h: ρ is the liquid’s density, g ≈ 9.8 N/kg (often 10 in calculations), h is depth. Pressure grows with depth and density and does not depend on the shape of the vessel or the amount of liquid; that is why submarines are built very strong. At the same depth the pressure is the same in all directions. Gas pressure arises from molecules striking the vessel wall; the more a gas is compressed or heated, the greater the pressure (a bicycle tyre). In communicating vessels a homogeneous liquid stands at the same level; for liquids of different density h₁ : h₂ = ρ₂ : ρ₁ — the column of the less dense liquid is higher. Communicating vessels are used in teapots with spouts, water towers, locks and fountains.
“Shape of the vessel”: fill transparent vessels of different shape (wide, narrow, bent) with water and see that in communicating vessels the water stands at one level. Safety: use plastic bottles rather than glass; do not spill water.