"Economists are going to add a twist here. A product is considered a necessity if it’s one where we don’t increase our purchase of it very much when our income rises," he says.

"So, for example, take water: When our income goes up most of us aren’t going to use a lot more water than we did when our income was lower," he adds. "So we say water is a necessity. You might say the same thing with milk and bread.

"On the other hand, a luxury is one where the opposite situation happens: When our income goes up, we purchase a lot more of it. So filet mignon steak would be a luxury for a family if when that family sees its income go up … they start eating a lot more filet mignon each month.

"So this indicates what a necessity and what a luxury is," he concludes. "It can differ, however, from person to person depending on their likes and preferences."