Kid Rolls Like a Boss in Custom Cadillac Stroller

They are made of tightly packed cylinders, they way coins pack on a table when crowded. And there are indeed little spaces between them, or there would be, but they get smushed together and filled in. You can see the varying thickness in the walls in the photo above.

This is sort of related. You might find it interesting...
http://www.digitalartform.com/archives/2009/06/simulating_voro.html

"If you cant show your work then the answer doesn't matter" - School Teachers. That always bugged me.