content: "Well, for this one I put a football down and built the arch around that as well as I could. Then I removed the football (carefully) and had someone else hold the shape while I filled the cracks with tiny rocks and sand until it could support itself. The key is the keystone, (the downward facing triangle piece in the middle) which any architect will tell you is required for an arch to support itself. They're actually really fun to tinker with and get frustrated with."