Sentence:  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 flame. With more settled people, animals were harnessed to capstans or caged in treadmills to <WSD>turn</WSD> grist into meal. 
Meaning of the word marked with <WSD> tags:  To change fundamentally; to metamorphose.