I was the first person in my school to actively play MtG. I mentionned it from time to time offhand with my friends, usually just something like "When I go to my Dad's, we often play Magic the Gathering and watch movies." turns out one of my friends played it as a kid, but didn't have his cards around.
About half a year later, he found some and we played a couple games. We're a core friend group of three, so the third guy quickly picked it up. I'm also part of another core group of friends (who eventually merged with the first because MtG). I got one hooked playing my decks, eventually got one for himself. One of his friends started playing a bit later.
Then one of my friends' younger brother took it up, and pulled in three more from his grade. Then some guy who left our school a couple years ago found out and started dropping by at lunches to play.
This year our lunch break is too short, but we used to have 9 regulars and 2 drop-ins. Starting with two guys, we quadrupled in size in about a year.
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That is how card games are introduced to an environment.