My daughter was making snickerdoodles (a kind of sugar cookie) this afternoon and, like me, she was adding ingredients before she made sure she had everything she needed. Sure enough, we were short cream of tartar. Now what to do, without making a run into the grocery store? I knew that cream of tartar was somehow connected to rising, so we looked in Joy of Cooking. No luck. Then we tried Harold McGee's On Food and Cooking: The Science and Lore of the Kitchen, and bingo! I found a whole page on leavening ingredients, including an explanation of how cream of tartar fits in.
The primordial leavening ingredient is yeast, but you don't always want that in a cookie, so there are chemical ingredients you can use instead. The most common one is baking soda; it is a base (or alkaline), and when it reacts with an acidic ingredient in the presence of water, they form lots of little bubbles that cause the bread or cookie to rise (think: vinegar and baking soda). This is why recipes that call for baking soda as the only leavener always have an acidic liquid in them: lemon juice, sour milk, buttermilk, even molasses or honey. I've used this info in the past to subsitute baking soda for baking powder by changing my sweetener from sugar to honey (it's not perfect if there is much sugar in the recipe, but it's better than nothing).
Lots of baking recipes call for baking powder, which is baking soda with an acidic salt added and only needs liquid to start the bubbles forming. Cream of tartar is an acidic salt, and in fact, you can add it to baking soda to get baking powder: two teaspoons cream of tartar plus one teaspoon baking soda is equivalent to 3/4 teaspoon of baking powder. Maybe not surprisingly, our recipe called for twice as much cream of tartar as baking soda, so we could substitute baking powder - except that the baking soda was already in the flour. So we cut back on the baking powder by the amount of the soda and crossed our fingers. It worked perfectly; if anything, the cookies are even better than the usual recipe. So we got good cookies, no grocery-store run, and a good chemistry lesson - not bad for an hour in the kitchen!
Looking for a link for vinegar and baking soda experiments, I also learned that you can use vinegar and baking soda to open sluggish or clogged drains: pour 1/2 cup baking soda down the drain. Add 1/2 cup white vinegar and cover the drain if possible. Let set for a few minutes. Then pour a kettle of 6 or more cups of boiling water down the drain to flush it. The combination of baking soda and vinegar breaks down fatty acids into soap and glycerin, allowing the clog to wash down the drain. (Do not use if any commerical drain opener has been used or is present.)