Category: doors and walls ...

going cross-country skiing right out your back door
a yellow brick road to your doorstep
two-way pet doors
the refrigerator door as an art gallery
going through a door marked "Authorized Personnel Only"
exploring the caves of Lascaux and their 20,000-year-old wall paintings
standing a top the Great Wall of China
scaling an indoor climbing wall
a secret bookcase wall that opens to reveal a storage room
tunneling through the blue walls of a glacier
the door to success - labeled "PUSH"
opening doors for others
doing the Soto Zen face-the-wall meditation, sitting 12-18 inches from a completely blank wall
inspiring art on the walls
Masters open the door, but it's up to you to enter. (Chinese proverb)