On Earth, the primary way our bodies lose heat is through conduction, where the atoms in our bodies interact with the atoms in the air or water, transferring heat energy. If you are in a situation where you are warmer than your surroundings, such as falling into icy water, you will lose heat much faster than your body can generate it due to the temperature difference.

In the vacuum of space, there is no air or water to conduct heat away from your body. Instead, heat loss occurs through radiation, where your body's warm atoms emit energy directly into space. However, this process is slow, and in space, the lack of oxygen would be fatal long before you would feel the cold.