This activity demonstrates air pressure. As the bag collapses, having part of the air inside the bag removed, the student will start to experience atmospheric pressure pushing in on them. The ideas that the atmosphere has mass and applies pressure is very difficult for younger students to understand. By removing a portion of this pressure, a student can experience a little of atmospheric pressure first hand. A fun extension of upper level classes would be to have students actually calculate the force being applied by the vac.
Some students believe that blowing takes the pressure with it.
For many young learners, air neither has mass nor can it occupy space. Many children have difficulty making the connection between paper airplanes and jet airplane flight. This activity allows students to experience air pressure first hand. Many textbook presentations begin with abstract concepts and terminology. Classroom presentations usually ignore student's prior views and ideas.