Two cold callers feature in this episode. Jennifer Langston from Ontario in Canada sent this message to [email protected]:

"My husband has just taken up cold water swimming and he'll swim in temperatures as low as 6 degrees Celsius. I worry that it's too cold for him, but he claims that 'swimming in cold water is good for you', which drives me bonkers. Can you tell us if there is any scientific proof behind this?”

Adam takes a trip to his local lido and asks the locals why they get a kick out of a chilly winter dip. Meanwhile, Hannah calls the Antarctic to talk to meteorologist Richard Warren about the perils of a frozen beard.

Our second cold caller, Sarah Dudley, asks why women get cold feet in bed. Thermal physiologist Heather Massey is on hand with the answer.

But when it comes to the natural world, other animals are masters of sub-zero living. Frozen Planet producer Kathryn Jeffs, from the BBC's Natural History Unit, explains why polar bears are perfectly designed for the Arctic. And we discover why Paddington Bear is better suited to Peru.

Presenters: Hannah Fry, Adam Rutherford

First broadcast on BBC Radio 4 in 2020.