User: With regards to a phase change, how do you know when a substance changes from a solid to a liquid?
Assistant: When the substance is cooled, it starts to melt. When the substance is heated, it starts to boil. This is because when the substance is cooled, the molecules in the substance are closer together, so they have less space to move around in. This causes the substance to become more solid. When the substance is heated, the molecules in the substance are farther apart, so they have more space to move around in. This causes the substance to become more liquid.