To root a Christmas cactus in water, you'll need a clean, sterile glass or jar, rooting hormone, and water. First, cut a healthy stem from the mother plant, being sure to leave a few leaves at the end. Then, dip the cut end into the rooting hormone and place it in the water. Make sure the water level is just below the cut end of the stem, and place the jar in a bright, indirect light source. After a few weeks, you should see roots forming. Keep the water changed regularly and be patient, as it may take several weeks for roots to form. Good luck!