I went to the State Fair today with DD15. My favorite part is the arts and crafts, and all the canned things.
My stupid question is, Why are all the cans displayed lid down?
It's easier to open that way, because the vacuum is at the bottom (which is currently at the top), because all of the product has settled at the top (which is currently at the bottom.)
Do you have any evidence for that? From a general physics standpoint, it doesn't make a lot of sense. The entire contents of the jar, whether it's the food or the small amount of air, is going to be under the same (negative) pressure. Unless the contents of the jar are very, very thick, the air (not vacuum) will rise to the top when you turn it right side up to open. I'm certainly not going to try to open a jar of pickles or strawberry jam without turning it with the lid towards the top.
Sorry, but that sounds like a piece of folk-wisdom to me.
When I make jam, I turn the jar upside down when I seal it. This helps achieve the vacuum seal on the canning jars. If I want to make the seal stronger, I also give them a water bath. The non-water bath works best with jams and jellies, because the contents are so hot. Also, I boil the lids, so they are ready to seal. As the jars, lids, and contents cool, a vacuum is created. You can tell because when you use the current canning jar lids, they have a "dimple". As the contents cool, a properly sealed lid will pop in once as the pressure inside the jar is reduced enough that atmospheric pressure pushes the lid inward. An improperly sealed jar will allow the dimple to move up and down, sometimes making a popping noise.
My guess as to the display, is partially to add another layer of protection from an accidental vacuum break, show off the contents better, but also I'm guessing that the lids have the owner's information on them.