A few years ago, my friend Donald donated his dad one of his kidneys. While of course donating a kidney to a living relative who is a match and needs one seems an obvious choice, it might raise concern depending on what the need for transplant was—will the donor, too, find him or herself in need of a kidney down the road and also needing a donor? Today is World Kidney Day: what do you know about kidney disease, kidney donation, and transplant—and of course, about what your kidneys do!