"Under Construction" Is the fourth episode of season three and the thirtieth episode overall of the animated children's series 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 portashrinker to shrink 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 Mrs. Li's third appearance and William's second.
- This is the second episode where the bus doesn't turn into anything (the first being In the Haunted House).