Depending on the subject, he could go to his text book, online ebook, etc. He could look at his review if he was given one. He could find other books relating to the subject that could potentially help. (Not much to go on, hope this helps c:)
Search for trustworthy sites. Try not to rely on Wikipedia, as anyone can change what's there. Same with any other "wiki" site.
Chronological............................................................
Sure, trees produce oxygen that help us breath and animals help with the production of food
