"Numbers are a human invention, and they’re not something we get automatically from nature," says Caleb Everett. Kwangmoozaa/iStock Once you learn numbers, it’s hard to unwrap your brain from their ...

Colours can add up, if you have synaesthesia, that is. Remarkably, people who associate colours with numbers can switch between the two when doing simple equations. Synaesthesia is thought to be the ...