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

[Q]: 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
[A]: What makes up the mass number of an atom?

[Q]: Passage: Soil is a precious resource. It allows us to grow food and the materials we use to make everything from the shirt you have on to the medicine you took this morning. Soil is made up of small pieces of rock that have broken down over hundreds, if not thousands, of years. Soil is also partly made up of the remains of plants and animals, and is home to many organisms, from earthworms to ants. But soil can be damaged by unsustainable farming practices and clear-cut logging. In this chapter, you will learn how soil forms, what it contains, and how to protect it. Answer: soil
[A]: What precious resource allows us to grow food as well as materials we turn into clothing and medicine?

[Q]: Passage: The rate of radioactive decay varies from one radioisotope to another. The rate is measured by the half-life. This is the length of time it takes for half of a given amount of a radioisotope to decay. Answer: half-life
[A]:
How is the radioactive decay measured?