For today’s lesson, let’s turn to Wikipedia for some information on the wheel:

The wheel is a device that enables efficient movement of an object across a surface where there is a force pressing the object to the surface. Common examples are a cart pulled by a horse, and the rollers on an aircraft flap mechanism.

I feel newly enlightened! But could we suppose for just a moment that if I’m researching the wheel on Wikipedia I am not already intimately familiar with an aircraft flap mechanism’s roller assembly?