How do animals understand the physical world?

The natural world is full of examples of animals interacting with their physical environment in surprising ways: capuchin monkeys crack open nuts with rocks; dolphins use sponges as 'gloves' on their rostra when searching for prey on the sea floor; and New Caledonian crows manufacture stick tools to pull grubs from logs (Figure 1). Deeper into the phylogenetic tree we continue to see interesting examples of behaviors of this kind, such as octopuses using coconut shells for protection, tuskfish breaking open cockles by hitting them against coral heads, and bees learning to pull string to gain out-of-reach food. These sophisticated behavioral interactions with their physical environment suggest that animals might have a deep understanding of their physical world at a cognitive level. In this primer, we review the performances of a variety of species when faced with tasks that probe their understanding of their physical world.