An axle is a rod or shaft that connects wheels and allows them to turn. Picture a toy car: that straight piece running between the front wheels (and another between the back wheels) is the axle. When you roll the car forward, the axle spins, and the wheels attached to it rotate.

Axles are one of humanity's most important inventions, making wheeled transportation possible. Without axles, wheels would just be round objects that can't do much. The axle helps turn them into a system that can carry heavy loads smoothly across distances. Ancient carts, modern cars, bicycles, trains, and even skateboards all depend on axles.

On a bicycle, you can actually see the axles: those metal rods at the center of each wheel that the spokes connect to. The wheels spin around these axles as you pedal. Car axles are hidden under the vehicle but work in a similar way, often carrying tremendous weight while allowing smooth turning. When mechanics talk about a car's front axle or rear axle, they mean the shaft connecting those wheels.

The phrase “grease the axle” means to oil it so it turns smoothly with less friction, though people sometimes use it to mean making any process work more smoothly.