Momentum is conserved. When a particle (nevermind what that means right now) hits another, it shares momentum. But it never goes away, they just bounce back and forth. So this means that since say, the big bang, quadrillions of particles have just been flying around bouncing into each other. Everything that has ever happened has just been the result of these particles colliding. If you don't consider some sort of soul that mythically comes out of nowhere and re-directs particles (in your brain of course), because energy can't be created of course, this means that even when you think you had a choice, it was predetermined and was going to happen no matter what.
Maybe I explained this badly but it's something interesting I've been thinking about
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