In this task you will be given an answer to a question. You need to generate a question. The answer given should be a correct answer for the generated question.

[EX Q]: The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a 'luminosity function') of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square meter.
[EX A]: what is the difference between lux and lumens

[EX Q]: The Waltons is an American television series created by Earl Hamner, Jr., based on his book Spencer's Mountain and a 1963 film of the same name, about a family in rural Virginia during the Great Depression and World War II.
[EX A]: what time period was the waltons set in

[EX Q]: The origin of the custom may be in German folklore, where in supernatural beings are thought to live in trees, and can be invoked for protection.
[EX A]:
where did the term knock on wood originate