For example, discussions of cameras, automobiles, computers, Dungeons and Dragons, and the SCA Board of Directors are obviously impossible for someone who has never heard of them. Perhaps less obviously, discussion of whether a particular song or style of clothing is "period", i.e. existed in the Middle Ages, is if not impossible, at least nonsensical for someone who lives in the Middle Ages. Likewise, asking someone "what century is your persona from?", while reasonable when talking about the Middle Ages, is ridiculous when trying to live in the Middle Ages. The sign at the entrance faces out; it is meaningless to the medieval people living within. For further discussion of this philosophy, see the various "Articles About Persona" and "Matters of Opinion" in Cariadoc's Miscellany.
At any rate, the inhabitants of Enchanted Ground have found over the
years that it's much easier to avoid talking about non-medieval things
if there are as few as possible non-medieval things in the camp.
One need not worry
about what euphemism to use for a Coleman stove if one doesn't have a
Coleman stove. And being surrounded by stuff that looks plausibly medieval
greatly eases the "willing suspension of disbelief" necessary to live in
Enchanted Ground. Therefore we avoid "obvious and unnecessary
mundanities", i.e. objects that an average, educated adult can
tell on sight didn't exist in the Middle Ages, with exceptions (hidden,
if at all possible) to meet modern standards of health and safety.
Some synthetic fabrics can be ignored from a distance (although why
anyone would want to wear synthetic fabric in August in
Pennsylvania escapes me); blue plastic tarps cannot.
Cast-iron pots (a slightly post-period technology, although most people
don't know that) are acceptable; Coleman stoves and Teflon are not.
Nonetheless, Enchanted Ground has always welcomed those who could not afford to invest in lots of reproduction equipment, either as visitors for an evening or as residents with borrowed equipment. The most important requirement, omitting obviously modern subjects from one's discussion, has the advantage of costing nothing at all.