According to those whose job it is to know such things, it only takes three weeks to become blind to the presence of stationary objects in our everyday worlds. Hang a new picture on the wall, and one is likely to notice it for about 21 days. After that it has become part of the scenery. It simply doesn't leap into the foreground any more. 

That's why it can be so hard to accomplish the simplest chores of housework before the arrival of guests. We've stopped noticing the screwdriver that's been sitting on the living room mantle for the last three months, or the pile of dirty shirts that has found a semi-permanent home at the end of the hallway. Housecleaning requires a new set of "eyes," eyes that are objective enough to see our surroundings as a discriminating guest would see them, not a…