A love thermometer refers to a toy known as a hand boiler, which uses a combination of temperature and pressure in an enclosed glass tube to appear as if the liquid boils just by placing hands on one of the ends of the device. Hand boilers marketed as love thermometers are novelty toys, and they cannot objectively measure love or the success of a relationship.
Love thermometers, which are technically love-themed hand boilers, use physics to create the appearance of causing liquid to float up and then boil in an enclosed glass container. The chemical within has a low point of evaporation, and as soon as a small amount of heat, even just that from a hand, is placed against the bottom of the device, the liquid begins to evaporate. With nowhere to go in the glass tube, the liquid immediately condensates higher up in the tube before evaporating again, causing it to appear to be flowing upward.
Once it reaches the top, the liquid begins to bubble as it constantly tries to go higher and become gaseous, and the heat at the bottom of the tube still affects it because of the hotter air in the enclosed environment beneath it. Because none of the elements can escape the environment, it continues until the glass tube reaches a temperature equilibrium, causing the liquid to flow back down and act as a normal room temperature liquid does.