The Hydro Flask 21oz (621ml) Lightweight Standard Flex Cap is perfect for adventures big and small. The Lightweight Trail Series is 25% lighter than our regular 21oz drinking bottles thanks to an innovative stainless steel design. Designed to deliver the same performance you get from HydroFlask, but without the weight, the Lightweight Trail Series makes it easy to take your hot or cold drinks on any adventure.The Hydro Flask drinking bottles are much more than a regular bottle, thanks to the unique TempShield insulation they keep the contents of your bottle cold for up to 24 hours and boiling hot for 6 hours.TempShield is the double wall, vacuum design of this unique bottle.