A nozzle squeezes a flowing fluid into a smaller area, so it has to speed up — and the pressure drops as it does. Follow the wide, slow inlet to the fast, narrow jet.
A nozzle is a passage that narrows. It does no pushing of its own — it simply makes the fluid pass through a smaller and smaller opening, which forces the fluid to accelerate. Anything that needs a fast, focused stream, from a garden hose to a jet engine, relies on this shape.
From the outside a nozzle looks like a tapering tube and nothing more. But its shape is doing all the work: the angles of the narrowing passage decide how much the fluid speeds up and how far its pressure drops.
At the inlet the fluid is held at high pressure but moves slowly, because the pipe there is wide and gives the fluid plenty of room. This is the calm, high-pressure side of the nozzle.
The body narrows steadily. Since the same quantity of fluid has to pass every second, and it now has less area to pass through, it is forced to move faster. The flow starts to stretch out and accelerate.
Speeding fluid up costs energy, and that energy comes out of the fluid's pressure. So as the velocity climbs, the pressure drops — the push is being converted into motion. At the narrowest point the fluid is fastest and the pressure lowest.
At the exit the fluid has traded much of its pressure for speed. It leaves as a narrow, fast jet at low pressure — a stream that can reach, cut or drive something precisely because it is moving so quickly.
It narrows the passage. Because the same amount of fluid has to pass through a smaller area, it is forced to move faster — this is the principle of continuity.
From pressure. The energy spent speeding the fluid up is taken out of its pressure, so as the velocity rises the pressure falls. Trade pressure for speed and you have a nozzle.
For the same reason a fast-moving fluid always sits at lower pressure: its energy is now held as motion rather than as push. The narrow throat is therefore the lowest-pressure point.
Anywhere a fast, focused stream is wanted — hoses, sprayers, jet pumps, water-jet cutters and rocket engines all use a narrowing passage to convert pressure into speed.
For a liquid, almost not at all — it is treated as incompressible, so slowing to a smaller area must become extra speed. Gases do compress a little, but the speed-up effect is the same.