Although they sometimes look a bit frightening, octopuses are amazing, and can do what seems like a magic trick! They can change the colour of their skin so that it's the same as the background behind them.
But they don't just change colour, they change the shape and texture of their surface as well, to make themselves look exactly like a stone or some seaweed, or whatever they want. When they do that, they disguise or camouflage themselves so well they almost disappear! This means they can hide from other animals that want to hunt and eat them, or sneak up on animals they want to eat.
See if you can spot the octopus in this video before it appears... brace yourselves, you're in for a shock! I promise it's not a special effect or a digital trick- it's a real video!
How do they do it?
Their skin is covered in little patches of different colours called 'chromatophores', each one of which has tiny muscles surrounding it. The muscles stretch the colour patches out, making them grow bigger. So if all the red ones get stretched out at the same time, the octopus turns red, and so on.
There are some octopuses that can act! They change their whole shape so they start to look like a completely different animal. Here's one actually doing it...
Octopuses are very intelligent. The scientists who look after them in aquariums say that you have to give them toys and puzzles to stop them getting bored and unhappy in captivity. Because they're so clever, they make great escape artists! Keeping them in a cage is difficult... being almost completely soft, and without bones, they can squeeze through any gap that's just a tiny bit bigger than the beaks they have for a mouth- the only hard part of their body.
Watch this one escape through a tiny hole!
A very clever octopus called Inky even mananged to escape from his tank at the New Zealand Aquarium. You can read about his brave adventure here.
In the past, people made up all sorts of stories about giant octopuses dragging sailing ships down into the water, but that was just made up. If anything, it's humans who are a threat to octopuses. Some of the ways in which people catch fish, called 'drag nets', destroy the ocean floor where the octopuses live.
Animals like an octopus are known as 'cephalods', a word that includes squids and cuttlefish. They all evolved from the same sort of animal as the snails you find in the garden.
You can find out lots more about octopuses here -