How many neurons are there in the human brain?

A neuron is a nerve cell that transmits impulses. Exactly how many neurons are in the human brain is a bit variable depending on the source. I have seen some sources stating that the human brain has 86 billion neurons, and I have seen other sources say that the human brain has up to 100 billion neurons. Either way, the human brain has lots of neurons. Billion is a word that I feel is used a lot without really understanding the sheer size of that number. I like to give students a way to visualize the size of billions. A million seconds ago was just about 12 days ago. A billion seconds ago was almost 32 years ago. Knowing that, it is simply incredible that approximately a 100 billion neurons are in the human brain.
http://faculty.washington.edu/chudler/what.html