All over sub-saharan Africa, menstruating women, pregnant women and sexual relations are seen as incompatible with productive activities (hunting, blacksmithing, cattle herding, agriculture, pottery, beer brewing, etc.). The article argues that there is a certain rationality or logic behind the prohibitions which is independent of the social contingencies found in the different societies, and which is linked with the fact that all these activities are metaphorically compared to human reproduction. The metaphors help us to understand mysterious and delicate biochemical processes, characterised by recurrent misfortune, but they also introduce a cognitive threat by creating similarity between things which are very different. The taboos then try to prevent the conjunction of the things metaphorically compared, in order to forestall the possibility of misfortune related to them.