To sanitize your flask properly before use they recommend following the steps in the Flask Care Guide. For the screwtop, you can allow it to soak in hot soapy water for several minutes. After sanitizing your flask, fill it up with clean water and allow it to sit for a few minutes. After allowing it to sit, pour out the contents in a clear glass to see if any particles are in the water. If there are still particles remaining after cleaning your flask, please repeat the sanitizing steps 1-2 more times.