Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: Three many families are known quarks divided into.
[EX A]: How many families are known quarks divided into?

[EX Q]: Rubber is an example of a(n) insulators that does not allow electrons to move through them.
[EX A]: Rubber is an example of what type of substances that do not allow electrons to move through them?

[EX Q]: Plane mirrors work because the light rays create a virtual image behind the mirror.
[EX A]:
Plane mirrors work because the light rays create what behind the mirror?