Growing like a weed

I read in a book awhile back (don't remember which book) the girl was saying "I don't know why anyone would compare their child to a weed" It seems so obvious to me- you pick a weed and you turn around and it's grown huge again. You just don't know how it got so large so fast! That is what my children are doing.. growing so fast that I can't keep up! Every time I turn around I swear they grew an inch!