Stainless steel is defined as a steel alloy with a minimum of 11.5% chromium content. The name comes from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel. 300 series stainless steels (of which water bottles are made) comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition of 18% chromium and 8% nickel, commonly known as 18/8 stainless steel is often used in flatware (and water bottles).
Facts taken from Wikipedia.