Out of buttermilk? For 1 cup, stir 1 tablespoon lemon juice or white vinegar into 1 cup milk and let it sit about 5 minutes.
1 cup buttermilk = 1 cup milk + 1 tbsp lemon juice or vinegar (rest 5 min)
Good for pancakes, biscuits and quick breads. It is thinner than cultured buttermilk, so it suits batters better than dressings.