When I want to blow my students' minds, I measure off the LJ WR, or the SP WR, or tell them that great sprinters top out at 26-27 mph and could get pulled over in a school zone . . .
Even though it's for a very short time, it's amazing to think that these guys run that fast. For comparison, the Tour de France record speed for an individual time trial is 33.9 mph (by Greg Lemond in 1989).