Indicate a noun phrase from given statement which mentions a pronoun.

Let me give you an example: this book introduced shakespeare to ovid; it was a fine selection of his writing.
The answer to this example can be: his writing
Here is why: in this statement the pronoun is his and his writing is referring to the quote in the sentence using the pronoun

OK. solve this:
this morning, joey built a sand castle on the beach, and put a toy flag in the highest tower, but this afternoon the tide knocked it down.
Answer:
knocked it down