Ever notice that food tastes different when it's hot. That's because heat brings out more of the aroma, and our sense of taste relies a lot on how things smell.
But, according to the February 2000 issue of the journal Nature, it's also because heat can stimulate taste buds, so that they send a signal back to the brain.
For example, if you put something warm on the front edge of your tongue, you'll get a sensation of sweetness. That's because the nerves that are being stimulated only know how to send one message to the brain: the sensation of sweetness.
The brain can't tell if the nerves were stimulated by a chemical, like sugar, or something else, like heat.
Cold temperatures, by the way, usually cause a sour or salty taste.