1. Melt the chocolate in a heavy-bottomed saucepan over a low flame with the water. Stir the chocolate until it is smooth. Let it cool slightly, perhaps 5 minutes or so.
2. Separate the egg(s). Beat the yolk(s) in a bowl. Slowly stir the melted chocolate into the yolk(s).
3. Whip the whites until very stiff and gently fold them over and over into the chocolate, so that they are perfectly blended, otherwise the chocolate may sink to the bottom. Place the mousse in ramekins or a lovely glass if making it for one or two, or a souffle dish if serving several guests. Check so that the mixture just about comes to the top of the dish, as nothing is sadder than a small amount of mousse hiding at the bottom of a huge glass bowl.
4. Refrigerate for 10 minutes if your craving is intense, 30 minutes if you have a little patience, or 3 hours or so if you're serving at a dinner party. Don't refrigerate it much longer, as this tends to make the texture too hard.