I want to know the difference of this sentence from just saying "That's a fire alarm sometimes". Why did he use "what a fire alarm"?
The phrase "that's what" something is is commonly used to describe the characteristics of how something is generally or what something is like. In the context of the video here, it would sound unnatural to say "that's a fire alarm sometimes". The word "what" in the phrase "that's what" is used to give the reasoning for something. Ex: Winter here is very cold. That's what winters in Canada are like. Ex: My dog is very high maintenance; that's what dogs are like.
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