Of Rainbows and Short-Term Memories

My front door faces nearly due West.  As a result, the setting Sun shines fully on the door and beams through my peep-hole.  This throws a circular rainbow on the opposite wall.

When Lyn and Mom arrived, Mom spotted the rainbow and showed it to Lyn.  Lyn was amazed and excited by it.  She asked what was causing it and we showed her the sunlight coming through the peep-hole.  

Each subsequent afternoon of their visit, Lyn noticed the rainbow and never remembered having seen it before.  She would become excited by it and we'd hear the same "Wow!  That's beautiful!  Mom!  Come look at this rainbow!"  Each day, Mom would come and dutifully look at the rainbow.  On some days, Lyn asked how it was happening and we'd explain it.  Some days, she did not.  

At no time, did Lyn show any indication that she'd had the same conversation previously.  After seven days, it was still new to her.