Ask somebody a difficult question, and chances are they will either look up at the sky or close their eyes.  What is going on there?

 Quite a bit of the brain is taken up with understanding what is going on in our sensory world.  For example, if you clasp your hands behind your head, most of the area taken up by your hands reflects the amount of the brain that is devoted to making sense of the information coming in through your eyes.

When your eyes are open, those areas of the brain that are involved in vision are getting input from the eyes, and this input keeps those areas busy.  Consequently, when you have to answer a difficult question or think about some visual memory from the past you either close your eyes or look upward to help you disengage from the world.  (Looking up helps, because the ceiling of the room or the sky are often much less visually interesting than what is happening at eye level and below.)

Those same areas of the brain are also involved in visual recollections of things that you have seen in the past.  It makes sense that the brain would re-use areas devoted to vision to help in memory for visual information.