As medium leaves, air enters. This air must pass through dense fibrous material or a membrane that will filter out microorganisms. The filter itself is sterilized as the reservoir and its connections are autoclaved. Organisms in the fermenter can grow back through the tubing. Yeast are big and slow and might not reach the reservoir in 24 hours.
Bacteria however can grow back through the tubing in a few hours.
An air break device prevents this. Again, since we are running our experiment for only 24 hours, we will not have to install an air break or a steril filter. Each group will conduct one experiment. You may devise an original experiment of your own. You can earn bonus points if you help the next group to run the experiment.