Becuase they beat most of thier compition flat. Boil them, and they hold up. They are still solid after having thier contents frozen, and they don't shatter until super, super cold (and your lips won't freeze to it). I have one that took a 35-40 foot bounce down the side of a small cliff.

What can't it do? It can't take going into a fire like metal bottle can, but other than that....