Our bottle is made from a hard, non-expendable material and is completely sealed. When you reach certain elevations, such as on a plane or hiking in certain regions, the atmospheric pressure will decrease making the air less dense, causing the volume of the air in the bottle to expand. This will make the pressure rise inside the bottle and can cause water to shoot out of the mouthpiece if opened suddenly.  

What we recommend is giving the lid a little twist of the full cap to release pressure inside the bottle, once you reach a high altitude, then close it again before opening the mouthpiece and drinking. 

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