What does to pick someone's knowledge mean

Explanation:

to pick someone's brains. phrase. If you pick someone's brains, you ask them to help you with a problem because they know more about the subject than you.

Since the words are not bolded, I'm guessing the words that are supposed to be in bold are: "You are such a bull in a china shop." 

If that us true (if those are the bolded words) means:
a person who breaks things or who often makes mistakes or causes damage in situations that require careful thinking or behavior

Im not sure what type of metaphor it is but im guessing it is an idiom (I dont even know if that is a type of metaphor)

Hope i helped