A Rosh Yeshiva recounts the following story: I was once on the plane and I had just received my meal, and I had opened the wrapper, whereupon I had a pressing need. I went to the bathroom and then I came back to my place.

As I was about to start my meal, I remembered the Halacha that a piece of meat that was left in a public place without "surveillance" can no longer be consumed. The Sages forbade it for fear that it was exchanged for non-kosher meat (the piece remains consumable if we recognize it or if it has a sign allowing it to be distinguished (see the details of this law in Yore Dea 63).

I knew that the piece of meat had certainly not been exchanged, but I remembered that the decrees of our Sages remain in force even in cases where the reasons no longer exist. It is absolutely necessary to respect the decrees of the wise even when we are sure that there is no risk! So, I closed the tray by pushing it to the side of my table.

My gesture caught the attention of the man next to me. "May I ask you why you are not eating?” he asked.

My attempts to dodge the true answer were not successful and I was forced to confess the truth. "The wise men of the Torah enacted (nearly 2000 years ago) a law prohibiting the eating of a piece of meat that has been "abandoned" lest it is traded for non-kosher meat. Hearing my answer, the man exclaimed: "Happy are the people who have such guides! I can only admit that the wisdom of your masters has saved you! Indeed, I saw that you received a kosher dish and when you got up, I wanted to taste it. Nobody looked at me and I exchanged one of your meatballs with one of my (non-kosher) meatballs..."

Indeed, happy are the people who have such guides! And happy he who will apply the laws of Hashem and the decrees of the wise without calculations!