Your hand entering the bag is just to simulate the mechanical forces within the cell - if you look at this image (https://en.wikipedia.org/wiki/Endocytosis#/media/File:Endocy...) it might help to visualize the mechanism in question.

It's literally impossible to perform the activity as specified in this lesson plan (in a normal classroom environment). Inserting your hand will cause some exposure of the inside of the bag to air.

Also: "Heresy kisses" -- Nice proofreading. Damn good thing this lesson plan isn't meant for students to view directly.

Is it too much to ask for commercially-supplied teaching tools to go through a basic quality control process?