Alton Brown taught me something (else) today. Baking powder contains cream of tartar (an acid, which by the way I found out earlier in the week was a left-over from the wine-making process) and baking soda (alkaline). The two balance each other out to produce just the right amount of gas to get things like, in this case, muffins to rise. In some recipes you need to add more baking soda to balance out an added acid such as buttermilk or chocolate. Neat, huh?
And I will remember this better because I'm typing it here. See, this is handy already!