I don't understand it.

I don't know why she does so.

I know a few things that are not my business, just as you know a little about shoemaking, that not being your business; but I don't understand them for all that." "Whose business is astronomy then ?" "Well," answered Hector, a little puzzled, "I don't see how it can well be anybody's business but God's, for I'm sure no one else can lay a hand to it." "And what's your business, Hector ?" asked Willie, in a half-absent mood.
Some readers may perhaps think this a stupid question, and perhaps so it was; but Willie was not therefore stupid.

People sometimes _appear_ stupid because they have more things to think about than they can well manage; while those who think only about one or two things may, on the contrary, _appear_ clever when just those one or two things happen to be talked about.
"What is my business, Willie?
Why, to keep people out of the dirt, of course." "How ?" asked Willie again.
"By making and mending their shoes.

Mr Dick, now, when he goes out to look at the stars through his telescope, might get his death of cold if his shoemaker did not know his business.