It's a well-known fact that human beings love the heat. Sunbathing under the scorching sun is popular, and sweating profusely in the sauna is considered to be good for the health. But that doesn't stop tens of thousands of Germans responding to the call of the cold: diving into lakes in November, splashing around in icy baths in the garden or hiking in their swimming costumes in the snow. They hope to de-stress, prevent infections or relieve rheumatic pain. But does it really work?