"It's only a biscuit"

Every individual use of the tin is small. Two digestives or a chocolate bar is nothing, and a slice of the cake someone brought in is nothing either. Anyone who made a fuss about one of them would look ridiculous, which is why you don't make a fuss, including to yourself.

The habit is made of small things, though, and it's worth adding them up once, without any alarm.

The NHS advises that adults have no more than 30 grams of free sugars a day. It describes that as roughly seven sugar cubes. Free sugars are the kind added to food and drink, together with the sugar in honey, syrups and fruit juice. The sugar that comes naturally inside whole fruit, vegetables and milk doesn't count towards the limit, and the NHS says there's no need to cut down on those.

To give you a sense of scale, the NHS points out that a single can of cola can contain as much as nine cubes, which is over the daily figure by itself. I'm not going to give you figures for particular biscuits and bars, because they vary and the packet will tell you. Have a look at a few. The label gives sugar in grams, and a cube is about four grams. On the Thursday in chapter 1 you had six biscuits and a chocolate bar between meals. I'd be surprised if that came to less than the whole day's thirty grams. It came on top of whatever was in your breakfast cereal, your yoghurt and the sauce on your dinner.

I'm not quoting that figure to frighten you. The NHS's own reasons for it are undramatic. Eating too much sugar can make you gain weight, it says, and can cause tooth decay. I make no promises about weight in this book, because what happens to anyone's weight depends on far more than biscuits. But the point about teeth bears directly on this habit, and it's less well known than it should be.

For your teeth, how often you eat sugar matters as well as how much. The NHS advice is to cut down on food and drink containing free sugars and to limit them to mealtimes. Each time you eat something sugary, the bacteria on your teeth produce acid for a while afterwards. If the sugar arrives with a meal, that's one acid attack. If it arrives at eleven, at quarter past three and at nine in the evening, your teeth get three more. Six biscuits spread across a day are worse for your teeth than six biscuits eaten as one pudding.

That's one of the reasons the rule in chapter 16 takes the shape it does. Moving sweet things to mealtimes is what the NHS recommends for your teeth, and it happens to be the change that removes the three jobs.

There's a second thing worth adding up, which is occasions. On that Thursday, sugar was part of three separate episodes in your day, along with the wanting beforehand, the verdict afterwards and the resolution on the way to bed. Add Friday's pastries, the Saturday cinema and Sunday at your mother-in-law's. However small the biscuits are, the habit takes up a fair amount of your attention across a week, more than the pleasure justifies.