AimTo compare the rate of heat flow into two vacuum flasks with vacuum envelopes of different materials (glass and stainless steel).DesignProspective non‐randomised comparison (n= 2).SettingHome (edge of the Great Sandy Desert), Sunday.Outcome measureChange in liquid nitrogen content over time.ResultsWithin eight hours, the flask with a stainless steel envelope lost almost all its nitrogen (about 800 g), while the flask with a glass envelope lost only about 100 g. The stainless steel flask therefore had a heat leakage rate about eight times that of the glass flask.ConclusionA glass‐lined vacuum flask is better for keeping liquid nitrogen cold (and coffee hot) than a steel‐lined flask.