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 friction out from between sticks to kindle a <WSD>flame</WSD>. 
Meaning of the word marked with <WSD> tags:  The visible part of fire; a stream of burning vapour or gas, emitting light and heat.