Batman's origin is first revealed in a two-page story in issue #33 (Nov. 1939). Batman became the main cover feature of the title beginning with issue #35 (Jan. 1940). Issue #38 (April 1940) introduced Batman's sidekick Robin, billed as "The Sensational Character Find of 1940" on the cover and the first of several characters that would make up the "Batman Family." Robin's appearance and the subsequent increase in sales of the book soon led to the trend of superheroes and young sidekicks that characterize the era that fans and historians call the Golden Age of Comic Books. Several of Batman's best known villains debuted in the pages of "Detective Comics" during this era, including the Penguin in issue #58, Two-Face in issue #66, and the Riddler in issue #140.