the chain rule and a teddybear

to difficult and yet so well known

The formula to find the derivate of a composite function, the chain rule, is for many students equivalent to the climbing of the Mount Everest. But strangly enough they don't realize that they are familiar with the logic of it. For, who never gave or go a present, packed into a box, whith a nice knit around it?
You can tell how to find the derivate and repeat it as many times as you want, learning is not hearing. A student has to understand. And whatever works is OK. Many times visual input or references to concrete things or situations can help to make the bridge to understanding. In teaching there's no limit on creativity.