I first heard this in 1961 but suspect that it has done the rounds a few times in various forms.

An instructor (possibly Mr Kennedy) was teaching a group of foreign students the complexities of something like astro-triangles or least-squares. One student said "there is one thing I not understand". The instructor was quite pleased as it meant that other things had got through, and asked which one thing it was. "No no" said the student, "there is not one thing I understand"!