Why would you do that? You have to put pressure on the cutting mat to cut the fabric, when you put pressure on the glass, what will happen? The glass will break and their will be slivers everywhere. So dangerous. Don't use glass for a cutting mat!!!! If your granddaughter can't afford a cutting mat, why don't you buy her one as a early Christmas gift or a birthday gift? I'm sure she will appreciate it. There are places where we quilters can cut corners, but using a sheet of glass as a cutting surface is NOT one of them.