I count the Prasna Upanishad among the most puzzling of all the principle Upanishads. It is a mixture of prose and verse, structured as a set of questions and answers. The questions are posed by six Brahmans to the "honorable Pippalada" and are typically staight-forward and interesting; however, the answers are often couched in obscure metaphors. One might make very little of this Upanishad without a significant background in the philosophy expressed in the broadeer Upanishadic tradition. Much of the language seems to harken back to earlier Upanishads.
There is a mildly comical passage in the third section, when one of the questioners asks six questions at once. Pippalada replies, "You are asking question excessively. But you are pre-eminently a Brahman -- methinks. Therefore I tell you."