The human brain is truly awesome.
A typical, healthy one houses some 200 billion nerve cells, which are connected to one another via hundreds of trillions of synapses. Each synapse functions like a microprocessor, and tens of thousands of them can connect a single neuron to other nerve cells.
In the cerebral cortex alone, there are roughly 125 trillion synapses, which is about how many stars fill 1,500 Milky Way galaxies.
These synapses are, of course, so tiny (less than a thousandth of a millimeter in diameter) that humans haven't been able to see with great clarity what exactly they do and how, beyond knowing that their numbers vary over time. That is until now.