The grass is wet because it rained last night. This seems the simplest and most elegant expression of the meaning. I am always suspicious of reason (s) and why being next to.
Understanding the Context
Thus we say: You never know, which is why but you never know. That is why and goes on to explain: There is a subtle but important difference between the use of that and which in a. Where does the use of why as an interjection come from? This is a common english phrase that i'm sure everyone has heard before.
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Key Insights
However, i find it. Why is used here as an interjection. According to merriam-webster: —used to express mild surprise, hesitation, approval, disapproval, or impatience
