How does thermochromic ink (both liquid crystals and leucodyes) work? I know that the ink becomes clear as temperature increases, but what's happening at a fundamental level?

im pretty sure that certain elements inside the compound sublime at x temperature, leaving only the clear half of the compound remaining

Thanks Vslayer. In that case, how does it return to its normal colour when the temperature drops? I'd have thought that if an element sublimed it would be lost permanently.