Octopus facts your brain refuses to forget

The octopus is what happens when evolution stops taking notes from everyone else. It has three hearts: two pump blood through the gills, one serves the body, and that third heart stops when the octopus swims, which is why an animal this capable prefers to crawl.

Its blood is blue, because instead of iron-based haemoglobin it carries oxygen with haemocyanin, a copper-based molecule that works better in cold, low-oxygen water. And the thinking is not where you expect: around two thirds of an octopus’s neurons live in its arms, so each arm can taste, touch and decide things semi-independently while the central brain handles the big picture.

No bones, either. A full-grown octopus can pour itself through a gap the size of a coin, which is why aquarium keepers tell escape stories the way parents tell toddler stories.

These facts are famous precisely because they are hard to believe, and hard-to-believe is easy to half-remember and get wrong at dinner. The deck below pins them down properly: each fact becomes a question, with distractors sneaky enough to expose the difference between “I read that once” and “I know that”.

Play it here, free, no account. To still have them next month, get the app: it brings each fact back right before it slips.