Asphalt is an amorphous, non-crystalline polymer compound. Its mechanical properties are determined by molecular motion and are significantly affected by temperature. At very low temperatures, the activation energy of asphalt molecules is very low, preventing free molecular motion. Asphalt is as brittle and hard as glass, a state often referred to as the "glassy state." As the temperature rises, asphalt molecules gain a certain activation energy, allowing the entire macromolecular chain to move. At this point, asphalt exhibits plasticity. As the temperature continues to rise, the activation energy gained by asphalt molecules increases, allowing for free motion and relative sliding between molecules. At this point, asphalt flows like a liquid, a state known as viscous flow. The temperature at which asphalt enters this viscous flow state is its softening temperature. However, there is no strict softening temperature for asphalt.