 During his time there he provided animation for many films, including the Betty Boop and Popeye the Sailor series, "Talkartoons", "Screen Songs" (with the famous "bouncing ball"), and the studio's first feature-length film, "Gulliver's Travels".In early 1939, Kneitel suffered a heart attack, and was absent from the studio until 1941.Kneitel returned just when Fleischer obtained the right to animate Superman.