“Sorry, I’m busy right now,” you tell a friend. That’s so true. For its size, your body is as busy as the galaxy.

Even when we’re resting and daydreaming, internal activity is nonstop. The brain, of course, is the crown jewel of our nervous system. It has 85 billion neural cells and 150 trillion synapses. These are its electrical connections, its possibilities. This figure is nearly a thousand times as great as the number of stars in the Milky Way.

The number of brain neurons is impressive. To count them at the rate of one a second would require 3,200 years. But the brain’s synapses, or electrical connections, are beyond belief. Those 150 trillion could be counted in 3 million years. And that’s still not the end of the matter. What’s relevant is how many ways each cell can connect with the others. For this we must use factorials. Let’s say we want to know how many ways we can arrange four books on a shelf. It’s easy: You find the possibilities by multiplying 4×3×2 — called “4 factorial” and written as 4! — which is 24. But what if you have 10 books? Easy again: It’s 10! or 10×9×8×7×6×5×4×3×2, which is — ready? — 3,628,800 different ways. Imagine: Going from four items to 10 increases the possible arrangements from 24 to 3.6 million.

Bottom line: Possibilities are always wildly, insanely greater than the number of things around us. If each neuron, or brain cell, could connect with any other in your skull, the number of combinations would be 85 billion factorial. This winds up being a number with more zeroes than would fit in all the books on Earth. And that’s just the zeroes after the 1, the mere representation of the number, not the actual count. The brain’s connection possibilities lie beyond that same brain’s ability to comprehend it.

All this architectural complexity may seem to lie inertly like a 3-pound lump of cheese with the same volume as a 1,400-cc motorcycle piston. Since the brain has barely more density than water, it does indeed appear to be a mushy, unimpressive lump. Its animation is utterly disguised. Making the brain vibrant are its relentless electrical activities. Unseen sparks fly everywhere. Each neuron functions on about 100 millivolts. A tenth of a volt is darned efficient. Even if you add up the brain’s entire energy consumption, it’s a mere 23 watts (for a typical person consuming 2,400 calories daily). Still, the brain uses a whopping 20 percent of the body’s energy despite taking up only 2 percent of the body’s mass. It’s an energy hog, and there’s no “off” switch; the current courses continuously.