To make pretend buttermilk, add 1-3/4 tablespoons of cream of tartar to a cup of milk, or add a tablespoon of lemon juice or white vinegar to a cup of milk and let it stand for 5 to 10 minutes. In many baking recipes, you can also just use plain yogurt or sour cream instead of buttermilk.
But it’s always worth remembering that you may have had a lot of fun on the days when you had a substitute teacher in class, but in the long run, it didn’t do you any good. You didn’t learn as much, or you had to work that much harder when your teacher returned. Substitutes are best employed sparingly.