When a body is not rigid but can spread under the action of forces — water in a glass, air in a tyre, mercury in a thermometer — we say it is a fluid. For fluids the question “how much force at this point?” loses its meaning: the right quantity is pressure, the ratio between force and the area it acts on. This section introduces pressure and the three fundamental laws of fluids at equilibrium — Stevin, Pascal, and communicating vessels — up to the instruments that exploit them to measure pressure itself.