Lead–lead dating is based on the assumption that the decay of uranium and thorium add the isotopes 206Pb, 207Pb, and 208Pb at a constant rate to the lead being sampled. Certain processes (e.g. pneumatolysis) can redistribute lead isotopes, however, altering the proportions in lead that is then isotopically anomalous. Anomalous lead can give dates that are older than the rock in which the lead is embedded, so it appears that nearby common lead will not form until some time in the distant future.