instruction:
Given a scientific passage and an answer, generate a question for the given answer.
question:
Passage: The mass number is the number of protons plus the number of neutrons in an atom. For example, most atoms of helium have 2 neutrons, so their mass number is 2 + 2 = 4. This mass number means that an atom of helium has a mass of about 4 amu. Answer: protons and neutrons
answer:
What makes up the mass number of an atom?

question:
Passage: An atom's protons and electrons carry electrical charges. Protons, with their positive charge, are designated p+. Electrons, which have a negative charge, are designated e. An atom's neutrons have no charge: they are electrically neutral. Just as a magnet sticks to a steel refrigerator because their opposite charges attract, the positively charged protons attract the negatively charged electrons. This mutual attraction gives the atom some structural stability. The attraction by the positively charged nucleus helps keep electrons from straying far. The number of protons and electrons within a neutral atom are equal, thus, the atom's overall charge is balanced. Answer: neutrons
answer:
What part of an atom is electrically neutral?

question:
Passage: There are many blood vessels in the kidneys ( Figure above ). The kidneys remove urea and other wastes from the blood through tiny filtering units called nephrons. Nephrons ( Figure below ) are tiny, tube-shaped structures found inside each kidney. Each kidney has up to a million nephrons. Each nephron collects a small amount of fluid and waste from a small group of capillaries. Answer: nephrons
answer:
What are the tiny, tube-shaped structures found inside a kidney called?