Why does she use "a" instead of "the" here?
Here she uses "a" to express the generality of going through a blackberry bush. Instead of using the definite article "the" to refer specifically to the blackberry bush in the video, she uses the indefinite article "a" to refer to blackberry bushes in general. She does this to express her idea that it is crazy for someone to want to walk through a blackberry bush, which is full of thorns.
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