So far we have treated fluids at equilibrium, that is, static ones. What happens when a fluid moves? The full answer requires an entire separate course, fluid dynamics; here we give only a taste of it, enough to explain a few classic phenomena, from the flight of an aircraft to the lifting of a sheet of paper. Two principles act as pillars: conservation of flow rate, which arises from the simple fact that fluid neither accumulates nor disappears, and Bernoulli’s theorem, which links pressure, velocity and height along a flow line. We close by observing where this ideal model stops working, that is, when turbulence comes into play.