The French historian Bulange a few years ago came up with the theory that all the geniuses of medieval France – artists, scientists, commanders – were born the following year after the country had gathered a good harvest (and, accordingly, pregnant and nursing mothers received enough nutrients to develop a full-fledged child). During the collection of evidence to confirm his theory, Bulange, however, faced unforeseen difficulties - a single accounting of the crop in France in the Middle Ages did not exist, the accounting books of the duchies and counties for the most part did not survive, and the records in the historical chronicles were uncertain ("in Aquitaine collected as many apples as not seen since the times of the world flood", but in the same year "in Burgundy all eaten bear and hammer").

Then Bulange came up with a trip around the archives in four parts of France and searched for menus and business books of old courtyards and hotels. It was accompanied by luck: it managed to find such a number of preserved old records to track the history of French lunches each year for centuries. But how do you know from the old menu the amount of harvest or non-harvest? After all, many courtyards and tractors had their own kitchen, and many dishes were not served to the table due to the harvest, but because of the craving of the cook.

The hero of the study was the vineyard. For centuries it went to the food of the poor and the rich, if the country had a shortage of bread, vegetables and meat. Relying on the appearance in the menu of dishes from grapefruit, Bulange concluded that in the years of his interest in France there were non-harvests, and the price of the grapefruit determined their scale. These calculations were subsequently confirmed by other indirect evidence and allowed the historian to prove the truth of his theory.