............................................................................................................................................

Dear Chris,

Why, indeed, should you brush your teeth? They, too, will be dirty tomorrow. Such arguments may impress a certain melancholy breed of philosopher, but economics offers clear advice. You cannot keep your car exactly as clean as you would want, so must balance the costs of washing it against the costs of having it too dirty.

Consider an analogous problem: how much money to keep in your wallet? Too much and you will suffer from theft or inflation. Too little and the wallet serves no purpose. Since it is troublesome to obtain cash, the rational customer will take out more than is ideal, gradually spend the money until there is less than ideal, and then replenish the wallet.

A graph of cash held over time would look like a sawtooth, regularly leaping up and sloping down. Now that we have ATMs it is easier to get cash, we do so more often, hold cash balances closer to the optimum, and the saw is finely grained.

The sawtooth model applies more widely. It explains, for example, why you should - as many people do - cut your hair too short, so that as it grows it is on average the correct length.

As for your car, you must find a way to wash it so that it is cleaner than you would ideally like, which considering your question should not be difficult. Consider a full service wash with wax and carpet shampooing. The car will be so extravagantly, embarrassingly clean that for a week or two after the wash you will be waiting impatiently for the dirt to begin accumulating.

Questions to economist@ft.com