We now know that thoughts are not controlled by blood vessels and sorrow has nothing to do with a "broken heart". They come from our brain -- the only thing which can try to understand itself. With a size smaller than a microwave oven, our brain contains more than 10 billions of neurons. Each neuron is connected to many (up to thousands) other neurons. The junction between neurons is called a synapse. Brain functions are the results of signal transmission through trillions of synapses.
I found this one by chance as I was trying to find ways to teach my 5-year old. Although it seems more academic (and not that well written in some cases) but I found it to be helpful.