You'll hear people say that potatoes are sprayed with a chemical that prevents it from growing. The idea being that you can keep a sack of potatoes without them sprouting. The other idea being that it keeps you from growing your own potatoes. I've gotten potatoes to grow in the past [link] so I knew that this should work again.
Yummy brought me a reddish potato with a touch of green in the eyes. I jammed a fork in either side, sat it in a cup of water, and left it. Roots formed quickly and grew like mad. After a couple of weeks the cup was mostly root. The green, however, had barely twitched. By "barely" I mean that some change had occurred. It was a tad more pronounced. As weeks passed the green grew a couple of millimeters, then a centimeter, and once it reached about an inch it had shaken off the growth inhibitors and took off. But rather than putting off runners, like I had expected, it grew up and up and up some more. Then it grew flowers. I hadn't seen potato plants with flowers before.
This picture was taken about a month ago. These flowers remain intact and have been joined by several others. They close each night and reopen each morning.