Stand up and raise both arms in front of you.  Open both hands with the palms facing down.  Make sure that both hands are at an even level.  Now, close your eyes.  Lower one arm to your side and then bring it back up to the same position.  Now, lower the other arm to your side and bring it back to the same position. Open your eyes and look at your hands.

With your eyes closed, it feels as if your hands are even,  however, when you open your eyes you will find that they are off by a 1-2 inches.

Now, repeat this test with your palms facing up.  You will find that you are much better at lining up your hands when they are palm up!

The reason for this mental mind trip is that we tend to use our eyes and our minds in balancing and measuring more than we realize.   Once you knew what the goal was, you engaged your brain into helping you accomplish the goal. Younger people and athletes without any thought tend to do better with this test because they have more body/mind awareness.  If you fail badly, it might be a sign that you need to get out and exercise more. If you practice this over and over, seeing the even arms with your eyes and thinking about the arms being level with your mind,  your body will gain the muscle memory to consistently make them even.