Many different parts of the brain work together to make you think. For example, when we look at a face, one part of the brain (called the “fusiform face area”) seems to be involved in identifying the person we’re looking at, while another (in the superior temporal lobes) is analyzing the person’s expression. Another part of the brain (in the prefrontal lobes) is involved in choosing between alternative courses of action. So if you think “Tom looks happy in that picture – I’ll vote for him” many different parts of the brain are involved in coming to that conclusion. 😉
Actually the question is much harder than I’ve made it seem because we don’t yet exactly what different parts of the brain do, and whether and how these different functions can be broken down into separate little chunks. But we do know with some certainty that different parts of the brain do different things and we have a fair idea of what some of those things are. It’s very exciting to work on this problem because there are so many new discoveries to be made.