How do I clean my flask?
The best way to clean your flask is to use boiling water and either a teaspoon of bicarbonate soda, or 2 denture cleaning tablets, and leave the mixture in the flask overnight.

After using the flask, we have found that adding a small dash of washing up liquid to the flask half filled with warm water, then shaking it will prevent the build up of staining. (Be careful when you open the flask after shaking, as pressure may build up, causing the contents to 'spit' when you open the lid.)