The Magic School Bus S03 E10 "Gets Planted"
The class is performing a play of Jack and the Beanstalk. As Phoebe gets stage fright, she decides to make the props. However, there is problem with the beanstalk: She tried to grow a real one, but it did not work; so she tried to build one, but that did not work either. Naturally, the Friz turns her into a bean plant and Phoebe becomes shorter than Liz. However, no one is sure how plants get food.