 Many physical objects have functions. (Stampe (1977) was the first to note this as a fact that might help causal theories of content.) A familiar mercury thermometer has the function of indicating temperature. But, such a thermometer works against a set of background conditions which include the atmospheric pressure. The atmospheric pressure influences the volume of the vacuum that forms above the column of mercury in the glass tube. So, the height of the column of mercury is the product of two causally relevant features, the ambient atmospheric temperature and the ambient atmospheric pressure. This suggests that one and the same physical device with the same causal dependencies can be used in different ways. A column of mercury in a glass tube can be used to measure temperature, but it is possible to put it to use as a pressure gauge. Which thing a column of mercury measures is determined by its function.