Put yourself in a position where you don't need to do anything and have no distractions.
Now, use your free will to choose to think no thoughts at all for just five minutes.
If you're like me or anyone I know (including heavy meditators), you won't be able to clear your mind. Thoughts will just ... occur.
The implications of this are many and profound. But even before considering what this means, take a second to note how powerful it is to notice the lack of control.
Most people assert they have free will because they can feel it. But experiencing thoughts thinking themselves despite our "conscious choice" is simple but indisputable proof of our lack of control over our thoughts. (More examples if needed.)
This is difficult to even consider, let alone accept and internalize. I've heard many smart people insist they have free will; on a podcast I heard recently, a brilliant person contended they "could not go on" if they didn't have free will. But of course they can. It is exactly like saying we know there is a god and we couldn't go on if there weren't.
I've said a great deal about the implications of recognizing we have no free will; I think this is the single best post. Personally, I'll be trying to draw on those insights in the coming days and weeks.
More on this in the "Brains!" chapter of Losing My Religions, which you can read for free.