In the oases of the Tunisian south, in Gabès, Tozeur, Nefta, Kebili and on Djerba, palm climbers cut a small wound at the crown of an ageing date palm and hang a clay pot under it overnight. By sunrise it holds lagmi: cloudy, ivory-white sap, cool and sweet, with notes of coconut water and fresh dates. It is sold in recycled bottles at the roadside and drunk the same morning, because the wild yeasts in it begin to ferment almost at once. By afternoon it is fizzy and sour, and by the next day it has become an alcoholic palm wine, also called lagmi, which is a different drink altogether. The common mistake is buying more than you can drink by lunchtime or leaving it in the sun: keep it on ice from the moment you get it. Fresh lagmi is ready to drink when it is sweet, smells clean and milky, and only a few tiny bubbles rise; once it tastes sour or prickles on the tongue, it has started to turn. It is raw and unpasteurised, so pregnant people, babies, older people and anyone unwell should skip it. Outside the oases, fresh coconut water is the closest stand-in. Chef's tip: chill the glasses too; lagmi is at its best ice-cold.