Think about the following questions:
1. Do you ever forget where you left your car keys?
2. Do you ever forget where you parked the car?
3. Do you ever forget where the grocery store is?
If you are like most healthy people, you answered "yes" to questions 1 and 2 but not 3. As we age, most of us occasionally forget where we've left the car keys or the car itself. This is annoying, but it is not usually a sign of serious memory impairment. By contrast, a person with normal memory almost never forgets the location of familiar landmarks like a grocery store.
Why? The answer to the riddle is quite simple: the grocery store generally stays where it is, while the car keys may get tossed in a different place every day. As a result, the memory of where you put your keys today competes with older, conflicting memories of where you put them yesterday, and the day before that, and every day before that.
The best way to solve this problem is to avoid it. If you have trouble remembering where you put your keys, find a place near your front door where you can hang or store them -- and then always put them there. It may take you a few days to get into the habit, but once you do, you will be freed of having to remember their location. Like the grocery store, they will always be in the same place.
Similarly, if you have trouble remembering where you parked the car, make an effort to always park it in the same area of the parking lot. After a few times, you will have learned the location, and you will never again have to do the march of memory loss around parking lot, hunting for your car.