I carry around a large backpack every day. It contains: a laptop; my purse (with wallet weighted down by nickels and pennies); a large nalgene water bottle, my lunch (generally in a heavy glass pyrex container), a binder for miscellaneous papers, a notebook, a datebook, my ancient cellphone, a million pens, and a cache (large) of sanitary supplies because you just never know. Also often: yoga clothes, children's toys and the childrens' snacks for school pick up. Combined weight: probably 30 lbs or so.
The kids' father carries around a bookbag. Contents: I don't know in detail but the bag is heavy. Probably 30 lbs or so of books.
Today Clara went to "work" in DC with her father for the day, and packed her own backpack. Contents: a 300 page volume about James Audobon which she can't read (it's a textbook) but whose pictures she is fond of; two blankets, several soft toys, all of her Pete the Cat books, a shoe box full of My Little Ponies and a pillow. Weight: too heavy for a little girl.
That apple didn't fall from the tree, did it?