Hip flasks are small, thin containers frequently made of stainless steel or silver. They have been around since the Middle Ages, and are still popular today. Small enough to carry around in your pocket, a hip flask is frequently filled with hard liquor and used to steal a quick sip in a public place or somewhere alcohol is impermissible.
Because of its narrow opening, hip flasks can be difficult to clean thoroughly, as you would a glass or wider container. However, by using a water and acid solution, a flask can be cleaned out quite well, and without using any harsh chemicals that could potentially harm you or the environment.
Pour one cup of water into the liquid measuring cup. Add one cup of the distilled white vinegar.
Mix the water and vinegar together in the measuring cup, using the spoon. Mix until the two liquids are thoroughly combined.
Boil water and fill the hip flask with the boiling water. Pour it out, and repeat one more time. Be careful not to burn yourself in this process, and hold the metal flask with a towel or pot holder if necessary to protect yourself.
Fill the flask about halfway with the water and vinegar solution. Close the top and shake the flask vigorously, for about 20 seconds.
Pour out the solution. Repeat Step 3 and Step 4, rinsing alternatively with boiling water, then the vinegar solution. After repeating these steps, Fill the flask with boiling water one more time, to clear out any residual vinegar in the hip flask. Turn upside down on a paper towel to dry. The water will come out and be absorbed by the paper towel.