Given a scientific question, generate a correct answer to it.

Ex Input:
When pumping blood, the hearts of animals with very long necks must overcome what force?

Ex Output:
gravity

Ex Input:
What do you call a device that produces a very focused beam of visible light of just one wavelength and color?

Ex Output:
laser

Ex Input:
Nonmetal structures contain what type of bonds?

Ex Output:
covalent