I was wiping down the kitchen counter. My right hand was sweeping along in that lazy figure-eight motion you don’t think about, heading straight for a mason jar. Before I registered the jar existed, my left hand reached over, picked it up, and held it in the air. The right hand kept wiping. Neither one asked me for permission.

I stood there for a second afterward, rag in one hand, jar in the other, thinking: what just happened?

Here’s the simple version, as best I understand it.

The first thing is the plumbing. My left hemisphere runs my right hand. My right hemisphere runs my left. They talk to each other across a thick cable of nerve fibers called the corpus callosum. So the question in the title has a literal answer. There’s a wire.

The second thing is that my brain saw the jar before I did. Vision doesn’t wait for me. It runs a little simulation of the next half second, all the time, whether I’m paying attention or not. Counter, rag, trajectory, jar. Collision. That prediction is what triggered the rescue, and it happened well ahead of the part of me that would have said “oh, the jar.”

The third thing is that I didn’t need to look at my left hand to use it. My joints and muscles are constantly reporting their positions upward, so the brain always has a live map of where every part of me is sitting in space. That’s why you can touch your nose with your eyes closed. It’s also why my left hand knew how far to travel and how hard to grip a smooth glass jar it wasn’t looking at.

The fourth thing is that I have wiped a lot of counters. Practiced movements get handed down out of the conscious part of the brain into the cerebellum, which handles timing and coordination and does not require my involvement. That’s the whole point. It frees up the part of me that gets to think about other things while the counter gets clean.

Which explains the piano.

I took lessons when I was about ten. I got the treble clef. I could read it, and my right hand found the keys, and that felt like real progress. Then came the bass clef, and I was done. I could not read two staffs and run two hands at the same time. It wasn’t the notes. It was that nothing had been practiced enough to sink down into the cerebellum yet, so both hands were still up in the conscious part of me, and that part of me can only really do one thing at a time. It was taking turns and calling it music.

I’m sure the rest of it was a ten-year-old brain still under construction, and a ten-year-old girl who would much rather have been outside playing in the dirt. Given the choice again, I’d probably still pick the dirt.

But that little jar rescue was four systems firing faster than I could notice, and my contribution to it was zero. It happens dozens of times an hour. Reaching for the coffee cup while reading. Stepping over the cat.

Mostly stepping over the cat.

I am grateful for a body that keeps the jars upright without asking me first.