Tonic water

Tonic water is a carbonated soft drink flavoured with quinine, the bitter alkaloid from the bark of the cinchona tree, and sweetened to make that bitterness palatable. Pale and faintly straw-coloured, briskly fizzy, and famously fluorescing an eerie blue under ultraviolet light, it began life as a way to take a daily dose of antimalarial quinine and survives as the defining mixer of the gin and tonic. It is non-alcoholic, caffeine-free, and prized for the clean, slightly medicinal bitterness that lifts a spirit rather than for any flavour of its own. Modern tonic carries only a trace of the quinine of its medicinal forebears, kept well below the level that once treated fever.