The average human brain weighs about 1.5 kg and contains around 86 billion specialized cells called neurons. Once we find a brain, weighing it is easy but if you want to count the neurons one by one and take one second per neuron, it would take more than 2700 years to do it! Since it is impossible to do it this way, how did we arrive at this number?
Back in 2009, a group of researchers from Brazil published the results of their work, where they (among other things) double-checked a fact that was common knowledge at the time – that the human brain contains around 100 billion neurons.
How did they do it? The researchers separated the brains into three parts: the cortex, the cerebellum, and everything else. The neurons were dissolved using chemical and mechanical processes to obtain a solution that contained only the cell nuclei (the part of the cell that contains the genetic material, the DNA). Because each nucleus represented one cell and the nuclei were evenly spread out in space, the authors could estimate the number of the cells in the brain by only counting a small fraction of the total cells. Using this approach, a task that would take 2700 years, could be done in weeks. When the researchers finished counting, they found out that those brains had on average 86.1 billion neurons.
While 86 billion and 100 billion intuitively seem very close to each other, you could fit an entire brain of a brown bear in those missing 14 billion cells! Turns out, it is always good to double-check your assumptions and intuitions!
For the curious reader, the original article is available here: https://doi.org/10.1002/cne.21974