Here's why:

* Changes the direction of effort: A lever lets you push down to lift a heavy object up, or a pulley system redirects your pull to lift something vertically.

* Changes the amount of effort: A ramp reduces the force needed to move an object to a higher level, while a gear system can increase the force applied.

Examples of machines:

* Simple machines: Lever, pulley, wheel and axle, inclined plane (ramp), wedge, screw

* Complex machines: Cars, bicycles, cranes, computers

Let me know if you'd like more details on any of these!