A dewar container—sometimes called a dewar flask or dewar—is a vessel with a vacuum space between a cold liquid and the outside walls of the vessel. One common type of dewar container is a thermos that you might use to keep drinks cold. Sir James Dewar developed liquid hydrogen more than 100 years ago, and in 1892 he designed a container to keep the liquified gas cold long enough to study. A dewar can be very simple, with double walls and an open mouth. Dewars built for specific uses can have features like lids, inlets, outlets, valves, and gauges.