Dave,
What comes to mind first would be the problems with simple scaling,
like why ants can get as big as we are no matter what the scifi films
suggest. You could have them compare two objects of the same shape and
material but different sizes say one twice as big as the other, and show
their volume is more then twice as great, by simply immersing them in water
in a graduated cylinder.
Hope this helps.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU