Start with a fact: an octopus has three hearts.

Two pump blood through the gills. The third pumps it around the body. And the blood is blue, because it uses a copper-based molecule, hemocyanin, to carry oxygen, rather than the iron-based hemoglobin in ours.

That's strange enough. Let's go deeper.

Level 2: The arms think

An octopus has around 500 million neurons — roughly comparable to a dog. But about two-thirds of them aren't in its head. They're in its arms.

Each arm can taste what it touches through thousands of suckers, and can carry out complex movements with surprising independence. An arm that has been severed can still reach and grab for a while.

Level 3: Escape artists

Octopuses are famous for escaping. They can squeeze through any gap bigger than their beak, the only hard part of their body.

In 2016, an octopus called Inky made headlines when he vanished from New Zealand's National Aquarium. Staff believe he squeezed through a small gap at the top of his tank, slid across the floor, and slipped down a drainpipe that led to the sea.

Level 4: They know who you are

Aquarium workers report that octopuses recognise individual people, behaving differently towards those they like and dislike. Some have been known to squirt water at particular staff members. In experiments, octopuses learn to open jars, navigate mazes and use objects — some species carry coconut shells to use as portable shelters.

Level 5: An alien mind on Earth

Here's the deepest part of the rabbit hole.

The last common ancestor of humans and octopuses was probably a small, simple, worm-like creature that lived more than 500 million years ago. Everything since — the octopus's big brain, its complex behaviour, its curiosity — evolved completely separately from ours.