Here's why:

* Thrust is the force that propels an object forward.

* To generate thrust, something must be accelerated in the opposite direction.

* A working fluid acts as the medium that is accelerated.

Examples of working fluids:

* Rocket engines: Use hot gas (like combustion products) as the working fluid.

* Jet engines: Use air (compressed and heated) as the working fluid.

* Propellers: Use air or water as the working fluid.

* Ion thrusters: Use ions (charged particles) as the working fluid.

The principle is the same: the propulsion system expels the working fluid at high speed, generating an equal and opposite force (thrust) that propels the system forward.