  In most experimental studies, active drops propel in a liquid bulk due to
self-generated interfacial stresses of solutal origin. Here, we demonstrate the
self-propulsion of a volatile drop on the surface of a liquid bath due to
stresses of thermal origin. Evaporative heat pumping is converted into directed
motion driven by thermocapillary stresses, which emerge on the drop surface as
a result of a symmetry breaking of the drop temperature field. The dependence
of the drop speed on the activity source, i.e. the evaporation flux, is derived
with scaling arguments and captures the experimental data.
