The reviving north Indian drink called chaas is thinner than a lassi. Fine sea salt is best here, because it dissolves easily in the cold yogurt.
Drink base: Whisk together yogurt, water, and salt in a bowl until smooth and salt dissolves.
Salty Lime Chaas: Stir lime juice into yogurt base. Pour into a glass, garnish with lime wedge, and serve immediately.
Sweet Lime Chaas: Place lime wedges, sugar, and mint leaves in a glass and smash with a muddling stick or the blunt end of a thick wooden handle until lime juice is extracted, mint leaves are bruised, and mixture is fragrant, about 1 minute. Stir into drink base and garnish with mint sprig; serve immediately.
Curry-Leaf and Cumin Chaas: Toast cumin seeds in a small skillet over medium-high heat, stirring frequently, until fragrant, about 30 seconds. Transfer to a mortar and pestle or spice grinder and coarsely grind. Heat oil in skillet over medium-high heat until shimmering, add curry-leaf stem, and cook, undisturbed, just until leaves darken slightly and curl at edges, about 15 seconds. Stir cooking oil, half of curry leaves, and all but a pinch of cumin into drink base. Pour into a glass, garnish with remaining curry leaves and cumin, and serve immediately.