Thanks to
xkcd's post about the Saturn V rocket, explained using only the thousand words most used in the English language, and Theo Sanderson's
Up-Goer Five text editor:
I work on problems people have with their ears. Some people lose their hearing as they get older, and this is what I study. People can lose their hearing for lots of reasons, like being hurt or being sick, but there can also be problems with how the body is put together.
Inside every person is the stuff that tells their body how to work. It tells the body how tall it should be, that sort of thing. Sometimes this goes wrong, and people have problems. There are many kinds of these problems, but the ones I study are the ones which make people lose their hearing when they get older. I would like to be able to find ways to help people so they don't lose their hearing.
... it's not that difficult, except that most of the interest and complexity involved - which I don't think is that hard to explain, but which requires more than the thousand commonest words - is lost. An interesting exercise, though :)