  We initiate a study of the gravitational-wave signatures of a phase
transition that occurs as the Universe's temperature increases during
reheating. The gravitational-wave signatures of such a heating phase transition
are different from those of a cooling phase transition, and their detection
could allow us to probe reheating. In the lucky case that the
gravitational-wave signatures from both the heating and cooling phase
transitions were to be observed, information about reheating could in principle
be obtained utilizing the correlations between the two transitions. Frictional
effects, leading to a constant bubble-wall speed in one case, will instead
behave as an ``antifriction'' force in the other and accelerate the bubble
wall. This antifriction will often take the bubble into a runaway regime,
significantly enhancing the amplitude of the heating phase transition
gravitational-wave signal. The efficiency, strength, and duration of the phase
transitions will be similarly correlated in a reheating-dependent way.
