A spam filter must decide: Inbox or Spam? You could write the rules yourself — or show the computer emails people have already sorted and let it find the clues. That second way is machine learning.

Try it: in part 1, switch rules on and try to get all 12 new emails right. Then open part 2.

Rules break on exceptions ("Free for lunch today?"). A learned filter weighs many small clues at once, and finds new ones when it gets new examples. It can still be wrong.