This is the sixth essay of a ten essay series in New Scientist on the nature of knowledge. The essay does not take a position one way or the other. Rather, it identifies three groups of animals that to a greater or lesser extent know things. One such group appears to include elephants, dolphins and chimpanzees. Such animals figure out certain things using inference, although it is not explained whether this is intuition or deduction with logic. The question, whether or not consciousness plays a part, is not mentioned. Another group, the example of which is the New Caledonian crow, is seen as using trial and error. The third group, humans, are seen as highly experimental, to the point of forming theories about how things work, massively assisted of course by having a superlative language capability.