Frank and Joe are mad at Charlie because they both tried to play an April Fool's joke on Charlie by setting him up to work on the dough-mixer. They thought it would be a super joke and Charlie would make a huge mess of things. As it turned out, Charlie excelled at his new job and the April Fool's backfired on them (25). This particular instance shows how mean people can be and shows that Charlie actually can learn something. Charlie gets to permanently work the mixer (26) and Mr. Donner even gives Charlie an advance (26).