The process when a liquid changes into a gas is called "evaporation" when it occurs at temperatures below the boiling point and "boiling" when it happens at or above the boiling point. During evaporation, surface molecules gain enough energy to transition into the gaseous state, often influenced by temperature and pressure conditions. This process is essential in many natural phenomena, such as the water cycle, where water from oceans and lakes evaporates into vapor, eventually forming clouds and precipitating back as rain.