Why does your mouth feel cold when you chew spearmint?

The reason for this is because spearmint contains menthol. Menthol triggers the same receptor as cold. It's something called the TRP-M8 receptor or nerve fibres in the mouth. It fools them into thinking they're colder than they really are. So when you breathe in, your mouth feels colder than it really is because it's activated this particular ion channel in the surface of the nerve fibre in your mouth. You think things are a lot colder than they really are. It's just a trick, a neurological trick, so you're not any colder really.