Life is Christocentric, whether men are aware of that fact or not.
I hope you got the irony of the cartoon. Calvin had to go flying off the cliff in the snow because that was his nature. And his free will didn't help him when he wanted to fly, but couldn't. His will was limited, just like ours is.
If your "demonstration" entails some gruesome story and the question, "Did God do this?" I will save you the effort. God did not "do it," but God permits evil to occur because it somehow plays a part in His plan for His creation.
Thankfully, we believe Paul who assures us that "we know that all things work together for good to them that love God, to them who are the called according to his purpose."
"All things."
Whatever you require: If Free Will was demonstrated to be true, would you accept it?