I am reminded of the story of the discussion at church camp for children where one of the counselors was leading a discussion on the purpose of God has for all of his creation.
They began to find good reasons for the clouds and trees and rocks and rivers and animals and just about everything else in nature.
Finally, one of the children asked, "If God has a good purpose for everything, then why did He create poison ivy?" This made the discussion leader gulp and, as he struggled with the question, one of the other children piped up, "The reason God made poison ivy is that He wanted us to know that there are certain things we should keep our cotton-pickin’ hands off of!"