Before using, make sure to not cross thread cap while screwing on or over tighten cap. For best results, hand wash the flasks and tube kits with mild detergent in warm water prior to using for the first and do the same thing before each use. Do not wash in dishwasher. It's a good idea to occasionally lubricate the cap and flask threads with with small amount of Beeswax, Silicone or Glycerin based lubricants; be sure to wipe excess. When not in use keep cap loosely tightened on flask. If over time you notice your flask "yellowing" this is normal due to the materials used to make the flask flexible. Because this material is somewhat porous it can absorb elements that may be in tap drinking water and sports drinks' ingredients. This is normal and is certainly not unhealthy or dangerous. We recommend using Bottle Bright and following their instructions to renew and restore.