Here's one to rattle your brain: I borrow £50 from a friend, then another £50 from another friend. So I have £100. I buy a shirt for £97, leaving me with £3. I give each friend £1 back, so I now owe them both £49 each. Which leaves me with £1. £49 + £49 is £98. And I only have £1 left? Which is a total of £99 Where's the other £1?