Ingredients: lime, soda water, flavoured syrup.
Cut a lime into quarters, squeezing each into a highball glass and dropping it in. Add ice and fill nearly to the top with soda water. Drop in a jigger of flavoured syrup (like those used to make Italian ices). My favourite is raspberry, to make a raspberry lime rickey.
If you don't have a lime, you can replace it with half a lemon and call it a lemon rickey!