_
It's hard to describe, but if you build the foam right, it holds it's own shape thru the forms engineering. Think of it like a ball, if you build a ball as a number of slices glued to themselves (which is how a foam ball is made) you can actually cut out both the bottom and top to make it look like a tire, and it will still have the rounded shape because of the slices themselves have a patterned or curved shape to them, they force the foam to bend and curve into this form. The glue only helps keep the form solid because it's attached to similar shapes that combine to form the whole. (Hope that helps, but man, my head hurts. I just need to write a book on the subject and sell copies to anyone who asks):lol