Your instructor hands you a closed flask partly filled with room-temperature water. When you hold it, the heat transfer between your bare hands and the flask causes the water to boil. Quite impressive! How is this accomplished?

The heat going from your hands heat the water in the flask. As is heated some molecules of water starts to change to vapor phase, increasing the vapor pressure over the water. When this vapor pressure equals the pressure inside the closed flask, the water starts to boil.