Sausage; very sharp clasp knife; and thin cotton or linen bag, with drawstring, for carrying the sausage, even on a motorcycle. We call the little bag a Wurst case (say it out loud and you'll see why). Frances makes them, and they last for years. Also: a plastic plate and a tiny mount of wine in a plastic cup. Feel free to use a paper handkerchief or nothing at all in place of the plate, and to drink water or even wine from the bottle if you wish.

The sausage plays a modest but admirably non-partisan role in countless World War Two movies. It is an essential part of the diet of the French Resistant; of the elderly German sergeant (no sympathizer with the Nazis he); and even sometimes of the British pilot being passed at unbelievable risk to his rescuers from safe house to safe house. It may be accompanied with a little bread, or even some raw onion, but it is so highly prized because it is one of the most concentrated forms of nutrition ever devised by man; or perhaps more probably by woman.

The sausage in question is not the British "banger", but the kind of air-dried sausage that starts out with more meat in it than its own weight. The skins are filled with fresh, raw meat, usually with a lot of fat; a good deal of salt and saltpetre to preserve them; and whatever herbs and spices are available and preferred locally. They are then hung up in a preferably cool, preferably dry place where because of the air drying they typically lose 10-20% of their weight. The clue is in the word "dried". Even "cool" and "dry" are however relative terms, depending on how brave, poor or hungry you are and where you live.

The result is an extraordinary concentration of calories in the form of fat and protein and (in the case of some sausages) even sugar.The basic idea of the sausage goes back thousands of years, whether you trace it to commercial butchers or to households who raised a pig or two of their own every year. Above the cheapest and nastiest, there is little correlation between price and quality. In France, three euros (maybe £2.50, $4) is the typical going rate for a decent sausage, with reductions for buying in bulk (10 euros for 4, 20 euros for 10).

They last for at least two or three months at room temperature: they do not need to be (and should not be) refrigerated. We keep ours in an old-fashioned meat safe in a cool room,preferably under 18C, 65F, but we don't worry if it goes higher, to 24C, 75F. With an already cut sausage, cut a thin slice off the end if it has gone dry and throw it away or feed it to the dog or the birds.Then start slicing for yourself. Like most French people, we prefer to peel the skin off ours before eating, except for the ones covered in cracked pepper.

Sausages are not something it is advisable to live on. The fat and salt alone assure that. On the other hand, if you are hungry or possibly starving, and if you don't over-indulge, there are few if any finer foods. Even if you're not very hungry, but are in need of a bit of a pick-me-up on a long journey, they are very hard to beat.They are also ideal if you have got up before dawn to take pictures and are cold and hungry while you are waiting for sunrise or for the Great Bustard (or whatever) to leave its nest. Baby wipes are good for cleaning your fingers if you don't want to get your camera greasy.

If at all possible, do not eat them the way they do in the movies. Forget the loving close-up of the clasp knife hacking off an inch (25mm) of sausage. That is more than enough sausage, but it will be easier to eat; taste better; and persuade your stomach that it has eaten more, if you slice it as thin as you possibly can. This means keeping your knife sharp. Thin slices are easier to chew and less greasy, and because they take longer to eat, your stomach (or more accurately your brain) will have more time to catch up with the fact that you have actually eaten something. In fact half an inch (12,5mm) can be more filling than twice as much, if you chew it thin slice by thin slice.

Sausages are of course salty: their very name comes from the Latin "salsicus" meaning "seasoned with salt". You will therefore need plenty of water either to help them down or after eating them. On the other hand, even if you are likely to be driving, a mouthful or two of wine (one or two ounces, 30-60 ml) is unlikely to leave you hopelessly inebriated or even, in most countries with a maximum permitted blood alcohol content of 0,30 or above, above the legal limit. Unless you are not a wine drinker it will also make the sausage taste much better. Little, and not too often, is the trick. Drink a tiny amount of wine and eat four to six slices of sausage every couple of hours and you can survive for a very long time indeed with a single bottle of wine and a typical 5 to 6 inch sausage (130-150mm) even if there are two of you sharing. Don't forget the water though, or you will suffer the torments of the damned.