No Picnic Today - Optical Illusion?

Dallas rarely gets heavy fog, so when it happened I decided to go out and see what presented itself.  This lonesome picnic shed on Lake Grapevine proved
to be an interesting and solitary subject. Normally you would see the lake and the far shore behind the shed. I found this view much more appealling.
Optical Illusion: A friend pointed out that if you stare at the top of the shed the lake appears and disappears. It worked for me. Your mileage may vary.