A blonde, holding a baby, walks into a drugstore a

A blonde, holding a baby, walks into a drugstore and asks if she can use the store's baby scale. "Sorry, ma'am," says the storekeeper. "Our baby scale is broken. But we can figure the baby's weight if we weigh mother and baby together on the adult scale, then weigh the mother alone and subtract the mother’s weight."
"Oh, that won't work," says the blonde.
"Why not?" asks the man.
"Well," she says, "I'm not the mother - I'm the aunt."