* Rocket Propulsion: Rockets work by expelling hot gas out of their engines. This expelled gas exerts a force on the rocket in the opposite direction, pushing it forward.

* Contact is Key: The expelled gas directly pushes against the rocket's nozzle, creating a physical contact point. Without this contact, the gas wouldn't be able to accelerate the rocket.

Non-contact forces would include things like gravity or magnetic forces, where objects can influence each other without direct contact.