As a physical chemist, I was amazed when it was discovered that the diatomic free radical, nitric oxide (NO) was intimately involved in the transmission of neurological information in mammals. Now a group led by Barry Trimmer at Tufts University has demonstrated that it is the key that turns on bioluminescence in fireflies. If you put fireflies in atmospheres that contain 70 ppm NO, you can make them stop flashing and begin to glow continuously. Not only does this paper illuminate (pun intended) some interesting biochemistry, but it also is a good example of how a scientific idea is developed through a sequence of experiments. A related story about the finding was written by Elizabeth Pennisi on pp. 2413-1414. Free Science registration allows you to read the abstract, but subscription is required for the full article.