“Which lake flows to this reservoir?” I asked. “I can't recall the name right now. It is a river,” replied my friend. “Do you know the difference between a river and a lake?” I said I didn’t. He too didn’t know. We both know many rivers and lakes because they have river or lake as suffix. However, we have never thought about the difference between the two words. We use most of the words without knowing the definitions. You may not know the difference between tortoise/turtle, explain/describe or moreover/furthermore, but you can understand the sentences which use those words. Most of the time, the exact meaning of a word does not matter since the meaning of the sentence which uses the word won't be lost. For most of the sentences, the word river or lake means body of water to me. All other shades of the meaning are irrelevant since I am neither a swimmer nor living near or associated with rivers or lakes. Consider a sentence: I can swim across any river easily! You will think you understand the sentence by taking the word river as body of water, but you will miss the nuance of the sentence, and you may not realise it. You may encounter a sentence which you can’t understand if you don’t know the distinction. Consider another sentence: I can swim across a lake, not across a river! Here the meaning “body of the water” won’t be helpful to understand the sentence, and you will realise you don’t know the exact meaning of those words. A swimming enthusiast or a fisherman may be interested in whether the water is flowing (a river) or standing (a lake), and they might have learned the distinction.