Everybody says you should buy what you need instead of what you want. Fine. But how do you tell which is which when you're standing in the shop holding it?

Someone offers you a free puppy. Free is free, right? Add up the first year and it comes to about $650 — and none of that was the puppy.

One person saves $25 a month. Another drops in $300 once and forgets about it. Ten years later one of them has seven times the other — and it isn't the one most people guess.

Ask any grandparent what a cinema ticket cost when they were your age and watch their face. They're not misremembering, and shops aren't getting greedier. Something else is going on.

The job pays $40,000. About $34,320 of it reaches your account. Nobody stole anything, nothing went wrong, and the gap is completely predictable — once you know there are two different machines taking a bite, not one.

A $60 coat, half price, and then another 20% off at the till. Almost everyone expects to pay $18. You pay $24 — and the shop has not cheated anybody.

Put a nickel next to a dime. The nickel is fatter, wider and heavier. Now guess which one is worth more — and get ready to be annoyed.