All material is made up of atoms and molecules. These are so small it is impossible to see them with the unaided eye. These atoms and molecules are in constant motion, even though an object seems solid to us. The molecules vibrate, gyrate, and move back and forth continuously. Thermal energy will increase in any object if the molecules are excited or caused to vibrate and move back and forth faster.
Some materials are good heat conductors.
This is the case with most metals. Other materials are poor heat conductors. This is the case with the ceramic you would find in a coffee cup. Describe a molecule. Explain why coffee cups are generally made of ceramic and not metal.