Thinking happens in your headbrain. Thinking is what made bacon happen, but also what made MTV. (Actually, thinking had nothing to do with MTV's creation)
Thinking is an extremely broad word. It ranges from an agnsty teen wondering why his girlfriend doesent like teenage mutant ninja turtles, *Cough*, to a baby wondering how to poop his pants. You're thinking right now! Feels kinda tickley, does it not? That's just because like a muhzillion (A zillion zillion) neurons are having a firefight inside your head. A massive electrochemical war, if you will. And all the parts of your brain are like bases for the neuron armies. And when one neuron army fires, something happens. Maybe its the motor skills base telling you to zip up your fly. Maybe its the sex drive base blindly firing everywhere. Right now its the Sensory* base firing at your eyes so you can read this.
That's pretty much what science says thinking is.
Most insane battles EVER!!!
So, if your brain looks like that mess, just imagine what Einstien's brain looked like! Mind-blowing, right???
*Sensory Neurons are a kind of neuron that respond the the stimuli of light, sound, touch, etc. Among other neuron variations, there are motor neurons, which cause muscle contraction and regulate glands, and Interneurons, which connect neurons to other neurons. (I.e. brian to spine.)