The difference between a useful source and a poor one depends to a great extent on your topic, purpose, and audience. On almost any topic you can imagine (Why do mosquitoes bite? Who reads fan fiction? What musicians influenced Sonic Youth?), you will need research—looking into sources, gathering data, and thinking critically about these sources—to answer the question. With most topics, in fact, your problem will not be so much finding sources as figuring out which sources to consult in the limited time you have available. Learning how to tell which sources are best for you allows you to use your time wisely, and taking effective notes allows you to put the sources to work for you.