A well-stocked refrigerator bar needs sour mix. My friends really like amaretto sours.
This recipe is pretty much exactly like the one from Serious Eats: http://www.seriouseats.com/recipes/2012/02/homemade-fresh-sour-mix-for-cocktails-easy-recipe.html
Yield: About 3 cups
1 c. Freshly-squeezed Lemon Juice (about 5 lemons)
1/2 c. Freshly-squeezed Lime Juice (about 4 limes)
1 c. Sugar
1 c. Water
1/4 c. Vodka
Squeeze lemons and limes through a fine strainer to get juice but not seeds or pulp.
Put sugar and water in a small saucepan over medium heat until sugar is dissolved. Let cool.
Combine juices, sugar syrup, and vodka in a storage container. Keep refrigerated. The internet says it will last a few weeks–and the vodka extends its life to about a month.