Something that I've considered for a while, and never got a really great answer to it, and still occupies my mind today. There are two ways the particles in our brain work; either they work in a predictably classical way, interacting with one another the way macroscopic particles would, bouncing off of each other and such, or they work in a quantum way, in multiple places at the same time and more probability than predictability.
Either way, we have no control over these particles. They're either random or predictable, but we don't start the reactions, we don't control the reactions, they just happen. With all this in mind, can we really say that we have free will? When our brain is thinking, it's the way it would have always thought based on atomic interactions. When we make a decision, it's the decision we would have always made because we'd always think the same things due to the interactions of the particles in our brains.
I know this is kind of a heavy topic and hard to dive into, but some of the interesting discussion I've had on the topic included whether there is something in the human body that's above the physical, whether the element of randomness involved changes anything, and whether we "start" the reactions and thus have free will in what to start.