I've been told that it's possible to root a Christmas cactus in water. It's a bit different from rooting other types of plants, but I've heard that it can be done. You'll need to make sure the water is clean and free of any contaminants. Then, you'll need to cut a section of the cactus off, making sure to get a piece with several leaves on it. Place the cut end in the water and make sure it's submerged. Keep the water clean and replace it regularly. After a few weeks, you should see roots starting to form.