The decision

Here's the rule, in one sentence. I have a glass of water with every hot drink and every meal, and at the flat hour I have a glass first and wait ten minutes before anything else.

That's it. It doesn't ask you to drink eight glasses, give up coffee, carry a bottle, count millilitres or want water. It asks you to double the drinks you already have and to change the order of one reach. If you follow it, you'll be at six or seven drinks a day by lunchtime plus the afternoon, your pee will be pale by mid-afternoon most days, and the flat hour will be shorter and the headache rarer. I'm giving you my expectation, not a finding. The rule hasn't been tested as a package. Its parts rest on the studies in chapters 3, 6, 8 and 12, and on NHS guidance, and the package is my arrangement of them.

Plans that name the moment work better than plans that name the goal. Peter Gollwitzer and Paschal Sheeran's meta-analysis of 94 studies found that "when X, then I'll do Y" plans had a medium-to-large effect on whether people did what they intended, across many kinds of behaviour. So here are three, and you can write your own.

When I make or fetch a hot drink, then I fill a glass of water and it goes where the cup goes.

When food is in front of me, then there's a glass on the table before I start.

When I notice the flat hour and feel the reach for the machine or the tin, then I drink a glass of water and give it ten minutes.

What to expect. The first week, more visits to the loo and a glass that feels like an instruction. The second week, fewer visits than the first and the glass appearing without a thought at least half the time. Somewhere in the second or third week, an afternoon where you get to four o'clock and realise the headache didn't come. Don't wait for that afternoon. Do the pale check, keep the count, and let the evidence arrive.

And decide it now, while it's clear, and not at three o'clock, when the tin is open and the flat hour is making the case for the biscuit.