eli5 Why do nipples get hard/erect when cold and also when turned on?

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Generally you are hot when turned on so it must be two different reasons

In: 28

For better ability to breastfeed in climates when dexterity is impacted by temperature. A babys mouth can more easily find a hard nipple. The arousal portion is just a side effect

The body is limited in how it can express the chemical reactions happening at any given time.

Cold makes nipples contract, thus making the area smaller and easier to keep warm. Nipples tightening during sex are respond to Oxytocin. Same effect, different catalyst.

Another example : sexual arousal and threat of violence have the same endorphin flood, tensing up muscles and making you feel hot as a result. Different causes, similar physical feedback.