We have a mechanical demonstration of random particle motion designed for use with an overhead projector.

E.M.E. Molecular Motion Demonstrator (see figure)
Collection of plastic beads of various sizes and weights (normally found with apparatus)
Overhead projector (normally in classroom)

This would normally accompany lectures on the properties of gases. It serves to provide a concrete image of moving particles in the "gas phase." According to the instruction manual that comes with the device, it can demonstrate every known phenomenon involving atoms and molecules. However, I find it most useful for showing that at any given temperature, particles of greater mass have smaller average speeds. This can lead into discussion of gas diffusion, isotope separation, etc.

The effect is demonstrated by first pouring a number of small identical plastic beads into the apparatus and turning on the agitation. Ask the students to note the average level of motion of the beads. Then add some much heavier (and visually larger) beads. The heavier beads will not move around as fast on average. Be sure to make the point that all beads are experiencing the same level of agitation or "temperature."

HAZARDS: This one is pretty safe.