You see a £97 shirt, but don't have any cash, so you borrow £50 from your mum and £50 from your dad =£100. You buy the shirt, have £3 change so give mum £1, dad £1 and keep the other £1 for yourself. You now owe mum £49, dad £49 so £49+£49 = £98 & your £1 makes £99 so where's the missing £1?