Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.
--------
Question: Question: Hardness, density, color, and melting and boiling points are all considered what types of properties? Answer: physical

Answer: Physical properties include color, density, hardness, and melting and boiling points.

Question: Question: Viruses were assumed to exist before they were first seen with an electron microscope in which decade? Answer: 1930 ' s

Answer: Viruses were assumed to exist before they were first seen with an electron microscope in the 1930s. Multiple hypotheses for viral origins have been proposed.

Question: Question: What is the energy that flows as a result of a difference in temperature? Answer: heat

Answer:
Heat is the energy that flows as a result of a difference in temperature. We use the symbol for heat. Heat, like all forms of energy, is measured in joules.