Flexi Says: When you look closely at the molecules of water, the molecules are relatively close together and they have a little room to jiggle. As you increase the temperature, the molecules continue to jiggle more and more, which causes an increase in the temperature. At some point, the temperature stops changing and it reaches a plateau. This is where the molecules have moved as far apart from each other as they possibly can. Any additional energy that is added to the system goes into overcoming the attractive forces the molecules have on each other. When the attractive forces are overcome, some of the water molecules are able to escape which results in the water boiling.