Sour mix, also known as sweet and sour mix or bar mix, is a mixer made of lemon or lime juice and sugar syrup. It is an important part of many cocktails.
To make your own, mix one part heavy sugar syrup (3 parts sugar, 2 parts water) with one part lemon juice, and add 2 egg whites per liter of mix. The egg whites are optional, but will make the drinks slightly foamy. You may want to adjust the sugar/juice ratio to give the mix the right balance of sweetness and tartness.