Tholins are simple organic compounds, the residue produced by running electricity through mixtures of Carbon (C), Nitrogen (N), Oxygen(O) and Hydrogen (H). Often they contain triple C-N bonds. This is important, because spectral analysis of Callisto by the Galileo spacecraft showed a strange absorption feature that looks as if it must be caused by triple C-N bonds on the surface. But small molecules like CN and HCN don't give quite the correct spectrum; only more complicated tholins seem to match of exactly with the data. This lead to the remarkable conclusion, in a recent paper [T.B. McCord et al, Science 278, No 5336, p.271, (October 1997)] that organic molecules are present on the surface of Callisto (and, to a lesser extent, Ganymede). This story was largely complete when these results were published, but the new information fit in very nicely with the premise.