  We present empirical data on misprints in citations to twelve high-profile
papers. The great majority of misprints are identical to misprints in articles
that earlier cited the same paper. The distribution of the numbers of misprint
repetitions follows a power law. We develop a stochastic model of the citation
process, which explains these findings and shows that about 70-90% of
scientific citations are copied from the lists of references used in other
papers. Citation copying can explain not only why some misprints become
popular, but also why some papers become highly cited. We show that a model
where a scientist picks few random papers, cites them, and copies a fraction of
their references accounts quantitatively for empirically observed distribution
of citations.
