20°C (68°F)
When humans and most other mammals start to get cold, there is a phenomenon called the pilomotor reflex which causes the tiny muscles called erector pilae at the base of each hair on the body to contract in such a way that the hairs stand more straight out of the body. For animals that have fur, this response serves two important functions. It traps a layer of air between the fur and the skin, serving as insulation, and since goose bumps can also occur in response to fear, it makes the animal look somewhat bigger, which might intimidate its would-be attackers.
For humans, who have mostly lost our layer of fur in the last few million years, both of these responses are vestigial, meaning that the response served a useful purpose for our ancient ancestors but doesn't do anything substantial to us now.
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