How to clean out a hip flask?

gypciesoul says:

Hot water and dish detergent. If you have a bottle brush… use that too. You can put the cap on and shake the bottle with the soapy water in it. Then rinse. Keep doing this until the rinse water is clear and clean. Drain the bottle upside down until dry inside. If it is not a glass flask, and is not new… then you might want to peer inside with a flashlight or something to make sure it is clean after you have washed and dried it.