You drop a block onto a box, and a toy pops out. If a baby was watching you, she could deduce that your action caused the happy arrival of the toy, because she understands cause and effect. She’d also realise that she could trigger the same event by placing a block on the box herself, because she can use her knowledge to actively shape her world.

These two abilities—understanding causality, and using that understanding—seem so simple and mundane to us that it feels weird to lay them out, and weirder still to separate them. But they are separate. That much becomes clear when you study an animal that can do one of these things and not the other.

The New Caledonian crow