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Becky Ripley and Emily Knight explore whether we can ever know what others know, and how we figure out if they're telling fibs.

Beneath the surface of the ocean, darting around in the dappled sunlight of the reef, you can find some of nature's most prolific liars. The cephalopods. Squid, octopus and cuttlefish; filthy con artists, the lot of them. They communicate with each other, and with both predators and prey, using dazzling patterns of shifting colour and texture on the surface of their skin. The messages they send can be sophisticated, but they're not always honest; males pretending to be females, octopuses pretending to be sea-snakes, cuttlefish on the hunt for love, pretending to have more innocent intentions. To deceive another, you might think, implies a level of understanding about what that other being knows, or is thinking. The philosophers call this Theory of Mind. But how much do the cuttlefish really KNOW about the tall tales they tell - and how much can we deduce about their intelligence as a result?

If cuttlefish are some of nature's best liars, let's meet some of the worst: human toddlers. Oh they lie alright, but they're terrible at it - they tell the most outrageous fibs that we can all see through. That's because they are just beginning to develop the complex skills of Theory of Mind for themselves, and they haven't quite perfected it. How they lie, and how they learn to do it better, gives us fascinating insights into the developing mind of a child.

Featuring Dr Jon Copley, professor of Ocean Exploration at the University of Southampton, and Dr Emily Jones, from Toddlerlab, at the Centre for Brain & Cognitive Development at Birkbeck College. Produced and presented by Emily Knight and Becky Ripley.