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Pour the sake you want to serve into a bottle with a narrow, tall neck and open mouth. You should not fill the container to the brim. Sake expands as it heats, and if you fill the container too high, it could burst out of the top. Fill a small saucepan with enough water to cover roughly three-quarters of the height of the decanter you are using for the sake. Place the saucepan on the stove and heat it over medium-high until the water begins to boil. If you want to be more traditional, there is a special utensil known as a "kan-tokkuri" you should use. If you decide to use it, heat the water in the stove with a saucepan or kettle and pour the water into the kan-tokkuri after it starts to boil. Turn off the heat and slowly submerge the sake in the hot water. Lower it carefully to prevent any water from getting inside the container. Let it sit, uncovered, for one to three minutes.  Place the sake as close to the center of the saucepan as possible. Do not allow it to tip or tilt as it sits in the water. For a more precise way of heating sake, you can measure the temperature of the drink using a thermometer to determine if it has reached your desired temperature. If you want to check the temperature of sake without the use of a thermometer, you can gauge it by looking. If small bubbles begin to rise, the sake is considered to be warm. If bubbles quickly and immediately rise to the surface, then the sake is hot. Carefully lift the decanter of sake out of the water and serve the hot drink immediately. If the container feels too hot to touch with your bare hands, wear oven mitts as you remove it from the hot water. You might also want to consider wiping the bottom dry with a towel before serving sake from the container.

Summary:
Pour the sake into a tokkuri or decanter. Boil water in a saucepan. Gently lower the container of sake into the water. Remove the sake from the water.