Derivatives. I imagined tangent lines traveling along a function curve and thinking 'I wonder what it looks like when we measure that?' And so I would try to visualize the changing slopes of the tangent lines at the same time. I also remembering wondering how to reverse it. Obviously didn't get farther than that, but I remember being very surprised when I took calculus and realizing that the mind game I had been playing was hugely important and widespread, and could in fact be calculated.