There was once a crab seller, who would export the best crabs across the world. What impressed the people that received these imports was how they were packed - a dozen crabs in a long cylindrical metal canister WITHOUT a lid! No one could figure out as to how the exact number of crabs as ordered were unerringly present in the canister. Finally one person couldn't hold his curiosity any more, and so traveled the distance to meet the crab seller. Mr. Curious asked the seller, "Sir, I have been purchasing your crabs all these years. Please enlighten me about your packaging technique. Every time we receive the canister, my team counts the number of crabs hoping that at least one would be missing, but alas! How, without a lid, are you able to send them with the confidence that all of them would reach the destination?" The seller smiling, picked up a shipping metal canister and tucked in about a dozen crabs that would just fill half the tube, set it aside on the deck and said, "just watch!" A crab in the canister started to crawl up the tube, but much before it would reach the top of the tube, the crabs from underneath would pull it down! Another crab would attempt similarly, only to meet the same fate. Only a package with just one crab would need a lid to keep it in!