One of the most difficult aspects of science is conceptualizing some of the unbelievably large, (and unimaginably small) numbers that routinely pop up. The Universe is 5.5 x 10^23 miles across. A human hair is about 7 x 10^-4 inches across. Hard to imagine how things like cells, proteins and atoms all relate to one another. Now, at least for the very small things, the University of Utah has developed a fun little Flash graphic to make sense of all of it.
The demonstration lets the user slide through different scales, ranging from the merely small, to the microscopic, to the atomic. It clearly illustrates not only the size of different objects relative to each other, but just how much that scale can make a difference in how an object behaves.
This is a neat little tool reminiscent of the classic short film The Powers of Ten, and will serve many students well as they transition from cellular biology to biochemistry and finally to organic chemistry. Personally, I always liked the metaphor that if a cell was a baseball stadium, a virus is the size of the ball, but I think this cool display works just as well.