Not all wheels and axles are used for rolling things along. Think about using a wrench to tighten a bolt, for instance. The circle you make by pulling on the wrench handle is the wheel. Where’s the axle? It’s at the other end, where the head of the wrench grabs the bolt.

You use effort to turn the wrench handle with a certain amount of force. The circle you make—the wheel—is larger than the axle. That’s the key. The larger wheel increases the force on the smaller axle.

The same kind of thing happens when you turn a doorknob. The knob is a wheel and is connected to a rod, which is the axle. The rod pulls on a latch that moves out of the doorframe so the door can open.

The knob is easy to turn. This wheel increases your force enough to turn the axle and open the door. But this amazing increase in force comes with a price, and that price is distance. You turn the doorknob a longer distance than you would have to turn the rod.

That’s the thing about simple machines: There are always tradeoffs. The larger the wheel, the less force you have to apply, but you have to apply that force over a greater distance. So, you trade distance for force. When it comes to doorknobs and wrenches, as well as faucets and screwdrivers, that’s not a bad trade!

Another fun way to enjoy wheels and axles is on a bicycle. A bike is loaded with this kind of simple machine. Of course, the big wheels with tires at the front and back are wheels and axles. So are the pedals. They are connected by an axle and form a wheel as they turn round.

Bikes also have toothed wheels called gears. A chain connectsa gear around the pedals with one or more gears on the back wheel. When you pedal, the chain turns the rear axle, so the rear wheel turns, too. The wheel is much larger than the axle, so the wheel turns a lot farther and faster than the axle. So does the front wheel, which just “goes along for the ride.”

Wheels and axles make work easier—and more fun! What other wheels and axles can you find around you?