How to Clean Your Flask: Baking Soda & Vinegar, Ice & Salt, and Denture Tablets. This video demonstrates two methods to clean various types of bottles, such as a thermos bottle, a thermos travel mug lid, a stainless steel coffee pot, a thermos flask, a smelly water bottle, a hydro flask without a bottle brush, a smelly metal bottle, a Yeti water bottle, and stains. The first method uses baking soda and vinegar, where one should put some baking soda and vinegar into the bottom of the flask and add hot water, then leave the flask to soak for a few minutes, scrub the inside with a bottle brush, and rinse repeatedly with hot water. The second method uses ice and salt, where one should fill the flask with ice, add a few spoonfuls of salt, close the lid and shake it vigorously, and then rinse out the flask with warm water. Both methods are safe and natural ways to clean stains and kill germs.