Learning math - (basic arithmetic)
Nothing like having a practical use for a subject to make learning it worth the effort!
I don't see first and second graders relating very much to cooking or quilting, though.
dunster's explanation was excellent.
One has to go by how many squares/pieces can be cut from a width. Most fabrics are only 42 to 43 inches wide "off the bolt" - and many shrink some, so one has to go with how many pieces per width, instead of square inches total. If one figures 40 inches "usable width" for estimating fabric needs, one will come out fairly close most of the time.
I checked fabric widths for about 400 pieces of fabric - most were in the 42-43 inch range. Including selvages.