My student: "As of now it is unknown, why...". I have very recently found out why. Do I tell him?
This sounds like an easy solution. You can give a couple of suggestions while stressing one as more likely to yield an answer, or even a single suggestion that &quot;may or may not work but could be an improvement on existing work&quot;. This is valid supervisory guidance, as it does not hold the student's hand but provides a direction to be explored and you are not providing a ready answer. The fact that you already know the answer is irrelevant. If the student is unable or unwilling to do so, this is not up to you. In fact, I think is it poorer supervision if you do not point out the weakness but simply let it pass.