SQUICK WARNING - I don't usually cover gross stuff here. But this question managed to send little spider feet across my forehead, so if "brain abnormalities" squick you out, move along.
There's a condition called hydrocephalus - water on the brain - in which the sufferer's brain development leads to much of the brain's cells being replaced with fluid. And yet, some patients still displayed normal intelligence, leading one researcher to wonder if we really need our brains to think!
This neuroscience entry suggests that, yes, Amelia, we DO need our brains to think, and here's why. And there are pictures. So, squick. But really, really interesting.