Colinsmum: He's actually just finished cutting a lot of teeth, so I'm thinking if it's molars, they must be some molars to be worse that what's just happened. 'course I thought exactly that about just about every tooth to date, so... (Also, I'm not sure that's really OT... but I'm a bit of a wholist ;))
I was just concerned that you might only be after thoughts relating to brain development :-) I'm very sorry, but my memory is that the two-year molars were indeed worse than everything that had gone before. (And teething took ages, and it was often quite hard to tell whether that was the problem or not, since by the time the tooth was actually palpable it seemed the worst was over. I was known to give him ibuprofen on suspicion of teeth, and with 20/20 hindsight I actually wish I'd done that more rather than less.) At least after them you get a break, and the next time teeth are an issue he'll be able to say so clearly!
DS used to like being given dictionary-style definitions for words, even words he clearly already knew. (I remember him saying, for example, "What is a bus?" when we'd just got off one, at about this age.) Might be worth trying that: "To play means... For example..."