Weighing a baby

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

'Sorry, ma'am,' says the clerk. 'Our baby scale is out for repairs.

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 lady

'Why not?' asks the clerk. 'Because,' says the lady, 'I'm not the mother -- I'm the aunt.'