Weekend Chatterbox

Flys is a plural noun. The singular form is fly. However, flys is not the plural form of every meaning of fly. It is only the plural form of fly in the sense of a zipper on the pants in the crotch area, a fly ball in baseball, an antique type of carriage, and the area over a stage.

The twins have on matching pants, and their flys are down on each of them. (zipper sense)
Flies is the plural form of fly in the sense of the winged insect. It is also the third person singular conjugated form of the verb fly, in the sense of an airplane, bird, or anything moving through the air.

My grandmother always flies on American Airlines. (verb form)