Re: How do the game peices that you sometimes see on cereal boxes work ?

This was not an easy one to track down.  The "decoder toys" actually are 
simple polarizers.  That's why certain rays don't pass through the film, 
even though they appear on the game piece.  To successfully duplicate the 
game pieces (by the way, the process appears to be patented, and none of 
the cereal companies would give me any information about them) you have to 
make the obscuring lines run in complimentary angles to each other, so that 
the polarizing lens blocks out those wavelengths, but allows the others to 
pass through.  They also suggested that the particular colours had 
something to do with it.  Sorry if this answer is a little vague, but the 
replies seemed to change whether I contacted American cereal manufacturers 
or Canadian.

Here are some websites you can check out for more information on the 
polarizing process that makes these "prizes" work.

"http://purcell.phy.nau.edu/courses/99/summer/SCI599/Participants/polarizat
ion/PolFilters.html"

"http://www.colorado.edu/physics/2000/polarization/polarizationl.html"

The last one has a fun little activity.