Another problem I have is explaining to my students what makes a fragment. Now, understand, if they've gotten to college and don't know what a sentence is, they aren't, um, necessarily absorbing the lessons well. But I find myself saying things like, "A sentence expresses a full thought," when you and I know that "I did" is just as much a sentence as "Charcot was the first neurologist, setting up practice in Paris in 1860 and serving as a major influence for Freud." So how would you define sentence and fragment? I throw up my hands and think if you have to ask, you'll never know.