The 21 oz is ideal for taking on the weekend bike ride or hike to keep your fluids cold. Or take it up the mountain and have coffee, hot cocoa, or soup waiting for you at the end of the day. The standard mouth features a larger opening than the original narrow mouth bottle. It is ideal for coffees, teas, mixing sport drinks, or just for those who need to take in more hydration, more quickly.Each Hydro Flask is cast from double wall vacuum insulated food grade 18/8 stainless steel, therefore they are BPA-Free and resistant to bacteria and odors. The double wall vacuum insulation technology allows Hydro Flasks to keep contents hot for 12 hours and cold for 24 hours.