NALGENE bottles are composed of several different varieties of plastic. The main chamber is made of polycarbonate, also known as LEXAN. This super-durable plastic is highly resistant to impact, staining, and odors. It retains these properties from -135o C (-211o F) to 135o C (275o F). It is also dishwasher-safe for easy cleaning. A microwave test showed that water could be safely heated up to coffee/tea temperatures without damaging the bottle. It was not tested until failure. The cap is made of polypropylene because it provides the best leak-proof seal for the top. It is also sturdy, but is somewhat weaker than the LEXAN and operates in a temperature range of -40o C to 135o C. This is the weakest part of the bottle and the only reason they ever break usually, although it can hardly be considered a failing. The only reason the cap can be considered ?weak? is because the LEXAN is virtually indestructible. It would be a rare occurrence indeed for a bottle to break under normal operating conditions, and it even holds up very well under extreme conditions (I have seen a bottle fall from a height of 30 feet, hit a rock on impact with the ground, and crack the rock without causing any damage to itself).