When you knock on a door, what causes you to hear a sound?

what is the answer

vibrations

Explanation:

When you strike the wood, energy is transferred from the finger to the wood. Some energy is lost during impact. The rest energy travels to the opposite end in the form of vibrations. At the opposite end wood starts vibrating and the air on the suface as well.

okay what's the question?

answer; people have no objective standard by which to judge themselves;