Why do you react when you touch ice?

When you touch ice, your body's thermoreceptors, which are nerve endings that detect changes in temperature, send signals to your brain to alert it of the cold sensation. This triggers a reflex response in your body, causing blood vessels near the surface of your skin to constrict in order to conserve heat and prevent further heat loss. This reflex response is what causes you to react when you touch ice, such as pulling your hand away quickly or shivering.

When you touch ice, the thermal energy in your finger flows into the ice, causing it to melt. The energy required to melt ice is called the latent heat of fusion, and it is a large amount of energy. As a result, your finger loses a significant amount of heat and begins to cool down. This cooling sensation is what you react to when you touch ice.