A flask can be a very convenient way to transport and keep drinks warm or cold while outdoors. However, flasks that are not cleaned properly after every use can breed bacteria and start to smell badly. This could also ruin the taste of the liquids that you pour into the flask later. Since many flasks are stainless steel, you will not want to use any detergents or other cleaners that can stain or scratch the flask.
Empty all of the contents from the inside of the flask. Put the flask's funnel into the flask's hole.
Pour 1 to 2 tsp of baking soda (depending on the flask size) into the flask. Add 1 cup of white distilled vinegar. Put the lid back onto the flask.
Shake the flask for approximately 30 seconds, and then empty the contents from the flask.
Fill the flask with tap water (using the flask's funnel). Shake the flask in order to thoroughly rinse the flask. Empty the flask and then rinse it at least one more time.
Spray disinfectant onto a clean cloth. Wipe the outside of the flask with the damp cloth.
Unscrew the funnel and wash the separate lid parts in your dishwasher or a basin full of hot water and washing-up liquid. Rinse thoroughly afterwards.