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

[Q]: Passage: Viruses , like the one depicted in Figure below , are tiny particles that may cause disease. Human diseases caused by viruses include the common cold and flu. Do you think viruses are prokaryotes or eukaryotes? The answer may surprise you. Viruses are not cells at all, so they are neither prokaryotes nor eukaryotes. Answer: viruses
[A]: The common cold and flu are examples of human diseases caused by what tiny particles?

[Q]: Passage: Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves. Answer: waves
[A]: What do wavelengths measure the size of?

[Q]: Passage: When iron and sulfur are mixed together in a certain ratio and heated, a chemical reaction occurs. This results in the formation of a unique new compound, called iron sulfide (FeS). A magnet cannot be used to mechanically separate the iron from the iron sulfide because metallic iron does not exist in the compound. Instead, another chemical reaction is required to separate the iron and sulfur. Answer: iron sulfide
[A]:
When iron and sulfur are mixed together in a certain ratio and heated, what do they become?