"Under Construction" Is the 4th episode of season 3 of The Magic School Bus. It originally aired on October 5th, 1996.
The class is going on a field trip to the new suspension bridge that's being built. However, there is a problem. Mrs. Li (Wanda's mother) needs Wanda to watch her little brother William until she gets off her business call. The class decides to watch him together. He, only a toddler, accidentally uses the Porta-shrinker to size down the entire class (minus Liz) and the bus and locks them in the bathroom. They uses the materials available to build structures to escape. This is the second episode where the bus doesn't turn into anything (the first being In the Haunted House).