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

Passage: An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another. Answer: variable
In a controlled scientific study, what do you call a factor that can take on different values?

Passage: Sources of voltage include electric generators and cells. Electric generators change kinetic energy to electrical energy. Chemical cells change chemical energy to electrical energy, and solar cells change solar energy to electrical energy. Answer: electrical
Electric generators change kinetic energy to what type of energy?

Passage: Two DNA strands link together in an antiparallel direction and are twisted to form a double helix. The nitrogenous bases face the inside of the helix. Guanine is always opposite cytosine, and adenine is always opposite thymine. Answer: double helix
What is formed when two dna strands link together in an antiparallel direction and are twisted?