What movie should you watch tonight? Personalized recommendation engines help millions of people narrow the universe of potential films to fit their unique tastes. These services depend on a machine-learning strategy called singular value decomposition, which breaks down movies into long lists of attributes and matches these elementsto a viewer's preferences. The technique can be extended to just about any recommendation system, from Internet search engines to dating sites.

For more on machine learning, read "How To Teach Computers To Learn On Their Own" in the July 2012 issue of Scientific American.

Illustration by Jen Christiansen; Interactive by Ryan Reid