Try dragging a heavy backpack across the floor, and it is hard and slow. Now put it on a wheeled cart and it glides along easily. Wheels are simple machines that make moving heavy things much easier.

A wheel turns around a rod in its middle called an axle. As the wheel rolls, only a small part touches the ground at a time. This means wheels cut down friction, so a load rolls instead of scraping along.

Because wheels lower friction, one person can pull a wagon full of bricks that would be impossible to drag. Cars, bikes, skateboards, and shopping carts all use wheels and axles to roll smoothly from place to place.

A pulley is another simple machine, a wheel with a groove that a rope rides in. When you pull down on one side of the rope, the other side goes up. A pulley changes the direction of your pull.

That direction change is a clever trick. It is much easier to pull down, using your weight, than to lift up. That is how one rope over a pulley can raise a flag to the top of a tall pole or hoist a heavy load.

You can even join several pulleys together to share the weight, so a heavy load feels light. Cranes, elevators, and the sails on a ship all use pulleys to lift things too heavy for arms alone.