Many people dont know, it has interesting origins. A long time ago, they used to ship manure (containing mostly you know what) on boats around the mediterranian. The cargo was always shipped on the lowest deck. Well, the boats in that period wore fairly leaky, and water would get in. When water and manure mixed, it produced methane. careless and unknowing crew members would carry torches at night, and if they went below deck when enough methane was present, the ship would explode. To solve this problem, they simply started shipping containers of it on the top deck. To make sure this was done, the manure containers were labeled "S" ship "H" high "I" in "T" transit.