Sentence:  Energy has seldom been found where we need it when we want it. Ancient nomads, wishing to ward off the evening chill and enjoy a meal around a campfire, had to collect wood and then spend time and effort coaxing the heat of <WSD>friction</WSD> out from between sticks to kindle a flame. 
Meaning of the word marked with <WSD> tags:  The rubbing of one object or surface against another.