When a solid is heated the particles gain energy and start to vibrate faster and faster. Initially the structure is gradually weakened which has the effect of expanding the solid. Further heating provides more energy until the particles start to break free of the structure. Although the particles are still loosely connected they are able to move around. At this point the solid is melting to form a liquid.
to summarize, ice cream melts because it absorbs the energy around it in the form of heat. This extra energy causes the atoms to vibrate, turning the solid into a liquid—and eventually into a gas
