Relate observations of diffusion to particle motion.
Students observed the instructor mass an orange, plate, and napkin. These items we identify as the mass system.  The instructor then cut the orange into slices on the tray and wiped the knife clean with the napkin, and then instructed the students to immediately notify the class  when they smelled the orange from where they were sitting in the room.  As they waited, the class observed that the mass of the orange was decreasing (scale reading).  Eventually students sitting closest to the orange smelled it first.  Over the course of a few minutes, the students farther away smelled the orange, too.   Students noticed that the mass of the orange continued to decrease.
All students did a storyboard of how they thought the orange particles diffused (defined in an earlier classroom discussion) throughout the classroom.  Storyboards indicated that the particles are mobile, and move through the air.  Even though the air in the room appeared still, the particles left the orange and traveled throughout the room.  Student ideas indicated they believe that the air particles in the room are in motion even though they cannot feel them moving, and that this is a result of particle collisions between the air and the orange forcing some of the orange particles into the air.
This demonstration and student work allowed students to add a new facet to the particle model:  The particle is mobile, and are moved through collisions with other particles.