For school I had to make a didactic model, that could be used as a teaching tool in one of my classes. We were free to pick a subject since it only was for practice, and here is the result.
It's a model I can use in my class to explain how the formula for calculating the surface-area of a circle came to be.
The original idea is not mine. This concept is very old. This is just my interpretation of it, adjusted to my wishes and I think I added some extra possibilities.