I am taking a course in quantum mechanics and a tennis lesson. These two courses seem totally unrelated. However, I found some related phenomena in both the micro-world and the tennis court. I hope the comparison below helps you to understand one of the most important notions in quantum mechanics.

In my class, different people have different approaches to learning tennis, which can be categorized into two major kinds. The first group of students, like me, can predict the position of the ball and hit it accurately. However, at the same time, I cannot control my posture while hitting, thus cannot hit back the balls in the direction that I intend. Meanwhile, the second group, like Crystal, focuses more on their postures, thus usually misses the ball.

It sounds like a real-life version of the Heisenberg uncertainty principle. In quantum mechanics, the Heisenberg uncertainty principle states through precise inequalities that certain pairs of physical properties, such as position and momentum, cannot be simultaneously known to arbitrarily high precision. That is, the more precisely one property is measured, the less precisely the other can be measured. Analogously, a tennis novice has a limited capacity to manage both their movement and posture, which separately influence their accuracy in hitting the ball and the direction of the ball hit back.

Through the analogy above, a measurement may be considered as a new hand of tennis. It cannot look after various aspects. The more it focuses on one side, the more it neglects the others.