PhD student Bradd Witt wants to learn about the changes to vegetation in Australia since widespread grazing by sheep started about 150 years ago. He has chosen an unusual way to obtain his data. Witt, from the Research School of Earth Sciences at the Australian National University, is studying the neatly preserved layers of sheep droppings under shearing sheds. By analysing the quantity of different carbon isotopes in the layers, Witt says that he can work out the amount of grass in a sheep's diet compared with the amount of shrubbery or other plant matter. The age of the droppings can be determined by the depth of the layer under the shed. Witt has been able to establish when grass species were overcome by shrubs and bushes. "I don't talk too much about my work at parties," Witt says.