Limes are distinctive for their green colour and refreshing, aromatic flavour. What are they? A small, sour citrus fruit related to the lemon, orange and grapefruit. What parts are used? They're used primarily for the flesh, usually as juice. The zest - the thin outer layer of skin - is also flavourful and can be used in the same way as lemon zest (in pastry doughs, cakes and savoury dishes). With at least one variety of lime (the kaffir), the leaves of the plant are used as often as the fruit. Types: many, but they're not all available in Asia. The most common (and cheapest) are the small, round Thai limes sold in wet markets, supermarkets and Southeast Asian grocery stores. Persian limes are larger and more oval in shape; they're sold primarily at supermarkets. Kaffir limes are expensive and not easy to find. These bumpy, unattractive fruits have very little juice but the flesh is wonderfully aromatic. The leaves are used to flavour Thai and Vietnamese curries, soups and stews. Fresh or bottled? Fresh juice is much tastier than bottled, which usually has an artificial flavour. What to look for: as with most citrus fruit, you want thin, pliable, unshrivelled skin and fruit that's heavy for its size. What else? They're a good source of vitamin C - like most citrus fruits. How to use: in ceviche, of course. This well-known South American dish has raw seafood (fish, scallops or prawns) that is 'cooked' by the citric acid of fresh lime juice. It's easy: just toss the seafood with plenty of fresh lime juice and season to taste with garlic, chillies and shallots or onions. Cover with plastic wrap and refrigerate, stirring occasionally, until the seafood turns from soft and opaque to firm and white, about eight to 12 hours. Don't let it stand too long or the seafood will become mushy. Before serving, toss the ceviche with fresh coriander, tomatoes and salt and pepper. Lime juice is essential in Thai and Vietnamese 'salads', where it's usually combined with fish sauce, sugar, chillies and garlic. In desserts, lime makes a nice change in dishes where you would normally use lemon juice and zest: in pound cakes (use the zest in the cake batter and glaze the baked cake with lime juice mixed with icing sugar), curds, sorbets and tarts. Bartenders use lime juice in many mixed drinks. Frequently, it is bottled juice but the drinks taste much better with fresh lime. Mix the juice into margaritas, mojitos, daiquiris and caipirinhas.