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Hot water bottles are usually relatively similar, no matter the brand, and consist of a thickly lined, flat, often rubber water bottle, with some sort of padding or cover on the outside. Some bottles may have a thicker cover made out of a different material so choose one that works best for you. Make sure that you do buy a water bottle with a cover, as you need some separation between the direct heat from the water bottle and your skin. Before you fill your water bottle, make sure the cover is already on the bottle. It may get a little wet, but if you try to hold your bottle without a cover while you are filling it up, the rubber may be too hot on your hands. Your water bottle will most likely already be in it's cover, and will have a plug at the top of the bottle that prevents water from spilling out. Start by unscrewing the plug so that you can fill it with water. If you water bottle has some water left in it, make sure you pour it out. You want to achieve the best heat from your water bottle, and using cooler, older water will make it harder for your hot water bottle to heat up. You can use water from your tap, but often times that doesn't get hot enough for your water bottle. However, boiling water from a tea kettle is much too hot for a water bottle. Try not to exceed 100 degrees Fahrenheit or 42 degrees Celsius with your water.   If you are using a tea kettle, you can let the water boil and then allow it to sit for a few minutes. This will provide you with hot water, but it won't be too hot that it burns your skin. Using water that is too hot can not only damage your skin, but it can decrease the life of you water bottle. The rubber that makes up hot water bottles can not withstand very hot water for a long time, so using water that is a temperature less than 100 degrees Fahrenheit is best for the life of your water bottle. Different water bottles have different temperature requirements, so consult the instructions for your particular bottle before using it. This step must be done carefully, because you don't want to burn yourself with hot water. If you are using a tea kettle, slowly pour water into your hot water bottle, allowing it to fill about two thirds full. If you are using a faucet, turn it off once it becomes hot, then line the opening of the water bottle up with the faucet. Turn it back on slowly, so that the water pressure doesn't spray onto your hands.  Make sure you hold the hot water bottle by the neck for the most stability. If you hold it by its body, the top may flop over before it is full, which will cause hot water to spill on your bottle and your hands. You might consider wearing gloves, or some other protection on your hands in case you accidentally spill some of the water onto yourself. You can also prop your water bottle up so that it stands up by itself by placing items around it to hold it up -- that way you can pour your water into the bottle without the risk of harming your hands. Once your bottle is mostly filled up (you don't want it filled to the top because you'll need a little space to squeeze out some of the air and a full water bottle may spill easily) slowly turn your faucet off. Then carefully remove the bottle from under the faucet, making sure you don't spill any of the water. If you are using a tea kettle, set the tea kettle down while holding the water bottle upright in your other hand, making sure you don't spill the water bottle or tilt it to the side. Make sure your water bottle is standing upright, with the bottom touching a flat surface. Then, slowly press the sides of the water bottle, expelling air from the bottle. Do this until you see the water in the bottle rise to the opening of the bottle. After you have expelled air from the water bottle, screw the topper back into the water bottle, making sure it is tight. Twist the plug until it can't be twisted anymore and then to test it, slowly turn the water bottle upside down to see if any water comes out. You may be using your water bottle to ease pain, or to provide warmth on a cold night. After you have filled it up, place your bottle on your body or in your bed and allow it to sit for 20 to 30 minutes. The bottle may take a few minutes to warm up, but shortly after you fill it, it should reach its maximum heat.  Make sure you don't leave the hot water bottle on your body for longer than 30 minutes. Direct heat for a long period of time can be damaging to your body, so you want to be as safe as possible. If you are using the bottle to ease aches and are still experiencing pain after, take the bottle off after 30 minutes and then replace it again after waiting 10 minutes or so. If you are placing your bottle in your bed, place it under you blankets 20 to 30 minutes before you crawl into bed. Then, when you go to bed, remove and empty out the water bottle. If you leave the water bottle in your bed while you sleep you run the risk of burning yourself or your sheets. Empty your water bottle out after the water has cooled and hang it upside down to dry, leaving the opening unplugged. Before using your water bottle again, check for leaks or damages by filling it with cold water. Do not air your water bottle out in an area that experiences temperature fluctuations (such as above the stove), under the sink, or in direct sunlight as these can decrease the quality of your water bottle.

Summary:
Choose your hot water bottle. Unscrew the plug from the hot water bottle. Allow water to heat up. Fill your bottle with water, about two thirds full. Remove your bottle from the water source. Squeeze air from the water bottle. Screw the plug back into the hot water bottle. Place the bottle on your desired area. Empty your bottle after use.