A blonde holding a baby walks into a drug store and asks the 
clerk if she can use the store's baby scale.

"Sorry, ma'am," says the clerk. "Our baby scale is broken. 
But we can figure the baby's weight if we weigh mother and 
baby together on the adult scale, and then weigh the mother 
alone, and subtract the second number from the first." 

"Oh, that won't work," says the blonde.

"Why not?" asks the clerk.

"Because," she answers, "I'm not the mother - I'm the aunt."