If my children are going to do one or the other, which is least harmful - drink or drugs?

As a GP I've seen far more kids ruined by drink, but that's because it has been more available. A few years ago, most young deaths from booze were in road accidents. Now binge drinking is causing deaths from liver failure, particularly in women in their 20s. That's horrifying, but my recent experience with drugs is even worse. We have hundreds of heroin addicts who started on tobacco, and teenagers damaged by 'E' and by cannabis who are under permanent psychiatric care. Many have criminal records, and some have died penniless, unloved and alone. On balance, it's easier to deal with alcohol abuse than with drug abuse, so if I had to make a choice I'd choose alcohol. But I despair at the thought of a teenager with either problem.

How much exercise should we really do each day to keep fit?

Before we civilized ourselves, as hunter-gatherers we walked and ran around 25 miles a day. Once the food was caught we ate until we were full, then slept it off until we were hungry again, maybe two or three days later. Our physiology hasn't changed since then, though our habits have. As long as we don't eat to satiation point at every meal we can get away with about an hour of brisk enough exercise to make us breathless at least four times a week. But if we eat too much and don't do those marathons in between, fat will inevitably replace fit.