sentence2_structure: Gases and liquids<>become<>solids
sentence1: A release of gaseous UF6 could occur only if there is a fire with sufficiently high temperature and duration to heat the solid UF6 to the point where it undergoes sublimation (change from a solid to a gas without ever becoming a liquid).
sentence2: Gases and liquids become solids at low temperatures.
question: When do gases and liquids become solids?