Basically, what are some mistakes that you can make while using the downward displacement of water method and how to fix them? This is actually a homework question and I'm not too sure what it means either. The original question:

State and explain Dalton’s Law of Partial Pressures. Why is it necessary to correct the pressure of a gas that is collected over water? {Background: Gases formed in a chemical reaction are collected using a technique called the downward displacement of water, feel free to research this technique before you respond to this question}