Answer: Brain

“Fathead” might be an old and not-so-nice insult, but it turns out that it’s not entirely inaccurate and also a bit ironic. When it comes right down to it, we’re all fatheads: the brain is the fattest organ in the human body and is compromised of approximately 60 percent fat.

The irony in the insult comes into play when you consider the purpose of that fat. Unlike body fat that is stored as food reserves, the fat in your brain is highly specialized. The brain is packed with a fatty substance known as myelin. The myelin serves as an electrical insulator that encases the axons of some nerve cells and protects them from each other the same way that the plastic casing of electrical wiring keeps them from shorting out. Fats, in addition to protecting our nerves like insulators, also serve as messengers within the brain wherein lipid messengers send signals between cells. It’s specifically because we are, well, fatheads that the human brain is such a marvel of cognition and processing.