Given a scientific passage and an answer, generate a question for the given answer.

[EX Q]: Passage: When heat is positive, the total energy of the system is increasing. Answer: it increases
[EX A]: What happens to the total energy of the system when heat is positive?

[EX Q]: Passage: Physical properties include color, density, hardness, and melting and boiling points. Answer: physical
[EX A]: Hardness, density, color, and melting and boiling points are all considered what types of properties?

[EX Q]: Passage: Wavelengths are described by a unit of distance, typically meters. Answer: distance
[EX A]:
Wavelengths are described by a unit of what?