Difference between 清水, 淡水 and 自来水
清水 clear water; 淡水 fresh water (contrast to salt water); 自来水 Tap water,  30 years ago most of the chinese people had to fetch water from nearby rivers, lakes or wells by using buckets (挑水). Tap water goes through pipes to homes, people think these water go to their home automatically (without 挑水), they created the word 自来水