(Fine. This is not the time to get into an argument about words. If she wants to call it magic, she can call it magic until she has access to the collected knowledge of humanity rather than some strange censored version.)

“All right. You could claim the sand and then reshape it into glass, but that would take longer because instead of just glass, you have sand grains with air and dirt between them.”

The mirror is moving its focus along the sand, gradually adding on to the edges of a disk of fused sand. It's not exactly window glass, but at least it's melted smooth on the top.