Attack of the 1000 Foot Veeblax! - S1.E22

Pleskit brings a Veeblax (a small, strange animal) to school. After it eats all of Tim's lunch, including bologna (no aliens eat meat), Tim and Pleskit deduce that the Veeblax just must be feeling ill due to gas. They figure out that they're wrong when the Veeblax starts to grow larger than the entire school.