Does this story sound familiar?

You stand at your own front door juggling an armload of groceries as you dig through your purse or pockets in search of a key. Finally you come up with one and shove it into the lock. It seems to fit, but no matter how you turn or jiggle it, the door refuses to open.

Frustration rises the longer you stand there fumbling. You resist the urge to hurl a few choice words at the door, knowing that they won't help...

...and then you see that other key on the key ring.

Maybe you feel a little silly. Obviously, it's been there all along. Why didn't you see it before?

When you slip the right key into the lock, the door opens easily.