On a foggy Saturday morning in 1953, a tall, skinny 24-year-old man fiddled with shapes he had cut out of cardboard. They represented fragments of a DNA molecule, and young James Watson was trying to figure how they fit together in a way that let DNA do its job as the stuff of genes.

As 2004 begins, staff writer Valerie Strauss asked students and teachers across the country to describe their most interesting or effective assignment so far this ...