SparkNotes, originally part of a website called The Spark, is a company started by Harvard students Sam Yagan, Max Krohn, Chris Coyne, and Eli Bolotin in 1999 that originally provided study guides for literature, poetry, history, film, and philosophy. Later, SparkNotes expanded to provide study guides for a number of other subjects, including biology, chemistry, economics, health, math, physics, and sociology. SparkNotes does not charge users but instead earns revenue from advertising.