Thermogenesis could be harnessed to prevent obesity, which makes it important to understand sources of heat loss to maintain normothermia. Trans-epidermal water loss (TEWL) through perspiration is a cooling mechanism for over-heated mammalian bodies, but also occurs at low ambient temperatures. Kasza and colleagues calculated the energy dissipation created by evaporative cooling through mouse skin and show that increased rates of evaporative cooling account for increased energy expenditure reported in mouse strains with deficient lipid layers. They also show that obese mice show lower trans-epidermal water loss. This raises the possibility that evaporative cooling could be a significant player in energy expenditure to avoid obesity.