Most copper heat pipes use water as a fluid. The key is that they only contain water, no air. As such, at room temperature, the pressure is very low. The pressure only gets to normal atmospheric pressure at 100C, or 212F.
The important point is that any enclosed space with only water, and water vapor, no other gasses, the water will be at its boiling point all the way down to near freezing. So at room temperature, any added heat causes the water to boil, creating vapor (which the transition from, for instance, 25C liquid to 25C gas requires a great deal of energy and absorbs heat).
The 25C gas created increases the pressure in the entire pipe, evenly and instantly, which causes the boiling point to go up. This causes vapor over the entire unheated part of the pipe to condense to a liquid, thereby releasing heat.
The fascinating fact is that the heat absorbed by the liquid to gas transition at one end, is released by a gas to liquid transition at the other end by the gas already at the other end, the heat is transferred by the pressure increase, not by the transfer of a heated material. Also, the heat transfer does not require the media to increase in temperature, only to change phase, which occurs at a constant temperature. Hence the very low thermal resistance.