Suppose we have four temperature readings and want their mean. We can calculate the answer by hand, then ask a computer to carry out the same steps. Beginning with a question we already understand gives us a result we can use to check our first program.
The examples run in the browser, so local setup is not a prerequisite. We keep one short temperature report throughout the chapter: first run it, then read its instructions in order, change one reading, and finally repair one misspelled name. Keeping the calculation fixed makes each new programming idea easier to see.
This chapter introduces only the language needed to describe visible behavior: program, source code, output, print, comment, error, and traceback. It does not try to explain every Python symbol at once. The next chapter returns to the same report and explains names, numbers, text, and assignment more carefully.