My favorite song is Let Me Ride, from Dr. Dre's studio debut The Chronic. (Jesus Christ, was that really 1994?!) There's one verse which I use to teach an important concept about necessary conditions... the fact that there can be many of them simultaneously. Seriously... I say this in class. It's a good one!
Dre's in his lowrider, picking up girls. (As usual.) He's on his way to a party, but he's got some important business to take care of first:
But before I hit the dope spot I gotta get the Chronic The Remy Martin and my soda pop
Count the necessary conditions:
1) The Chronic. Obviously it's not a party without that.
2) The Remy Martin. Classy dude.
3) My soda pop. Oh, he's cutting it with soda? If I were him, I'd buy cheaper booze.
So here's the LSAT lesson: All THREE of these things are necessary. The failure of any one of these conditions can keep Dre from the party. No Chronic? No party. No Remy Martin? No party. No soda pop? No party.
And it's really important to understand that these three things are necessary, but not sufficient. Even if he does have all three of these things, Dre still might not go to the party. There could be other, unmentioned, necessary conditions: Perhaps he's also got to swing by and pick up Snoop Dogg, for example. And he definitely has to avoid getting gatted on the way. Etcetera. So all we know for sure is that if he does make it to the party, he'll be packing all three of these things. If he's lacking any of them, he won't be there. And even if he does have all three of them, he still might not make it. That's what "necessary" means.