The brain has the greatest cluster of neurons in the entire body. A tiny slice of brain tissue the size of a grain of sand contains about 100,000 neurons. They are packed so tightly that a pebble-sized chunk of tissue from the human brain contains about two miles of neuron material. Your entire brain contains some 100 billion neurons, each one a fraction of a millimeter in size. To give you an idea of how many neurons this is, if you were to count to 100 billion, second by second, you would be counting for nearly 3,171 years. If you could stack 100 billion pieces of paper, the stack would be
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