Like Beardy, I did nearly all my uni work in pencil (much to the chagrin of my Director of Studies). When you're doing sums (or trying to do TRICKY sums), it's so much more reassuring when you know that if you cock it up a bit you won't have to resort to (i) starting again, or (ii) unleashing a 'woolly bear' (what my schoolmate James used to call a crossing-out).