Full Circle

It was one of those moments when I was trying to explain something to them and they ended up explaining something to me.

We're in the middle of a unit on volume and exploring prisms, cylinders and cones.  I was inspired by James Tanton's ability to explain things by getting at their essence. As if to say, "we can call a cylinder a 'cylinder' but it's just a prism made of circles--or a cone can be called a 'cone' but is it really any different than a pyramid?"

It was one of those, sitting around a campfire moments.  We're using stacks of paper and stacks of CDs to demonstrate why calculating the base area is critical because the rest of the solid is just like a stack of that area and no matter where we slice the solid, we get the same shape--over and over again.

Then comes the question about the cone.

The base is a circle but when you slice it, you get a...circle?  Wait, but it's a different circle.  Waitaminit. What about a pyramid?  Triangle base and when you slice it, you get a triangle.  But a different one.

Are the triangles related?

"They're similar.  Hey wait, this is a dilation."

And the tip of the pyramid is the center of dilation.

We did dilations in Unit 1.  This was a callback I didn't anticipate:  A pyramid is like a 3D representation of a dilation.

Thanks, kids.  I'd never thought of it that way before.