The Case of the Missing Pound
Once you've bought the t-shirt, you have £3 left over. You give £1 to Mom, £1 to Dad and keep the last £1. You now owe your Mom and Dad £49 each. £49 owed to Mom + £49 owed to Dad + £1 you kept = £99
Where's the missing £1?
(Hint: it's a mathematical fallacy)