Glass is great if you need a palette for acrylics or if you're cutting out a paper pattern with an exacto knife but a rotary cutter is just too dangerous to take a chance of having it slip and give her a very serious injury. Just think of how many people cut themselves using a rotary mat and imagine how much worse it could have been if the cutter had been sliding on glass. I wouldn't even take a guess how fast it would dull the blade.