Relationships can be difficult,” I remarked the other day to a friend who constantly has problems with her boyfriend. “Just remember to share. To play fair.”

“And,” I continued, “don’t forget to clean up after your own mess.”

She quipped that I sounded like our third-grade teacher, Mrs. Levy. (I figured it could’ve been worse — she could have told me I looked like Mrs. Levy. Then I might have to rethink our best friendship.)

Meanwhile, although I didn’t mean for her to take this advice literally, I realized that a lot of the recommendations about relationships I had been dishing out lately could have easily derived from Mrs. Levy’s remarks about common sense in the third grade.

All of it, of course, except for the part about remembering to wash your hands after going to the bathroom.

In any case, I compiled a short list — “Everything I needed to know about relationships I learned in the third grade” — in honor of my friend, and to give thanks to a little bit of Mrs. Levy’s wisdom:

• The best way to find out if your crush likes you is to have your best friend ask your crush’s best friend. You’d think that by the time we reach college — an age of independence — we would have gotten beyond the basic third-party (and third-grade) “negotiation situation.”

But when questioning if that cute boy in your orgo lab feels the same chemistry you do, your best bet is to result to primary-school politics. Enlist your best friend to begin a thorough inquisition — not of your crush, of course. Have your best friend find your crush’s best friend, and ask away.

• Find out your crush’s favorite play spot and ask to play along. In kindergarten, this meant sharing the sandbox; in college, this might mean sharing the sand along Lake Michigan.

Either way, as a colleague of mine confirms, the more you show up on her turf, the more likely you are to strike up a conversation. This can inevitably lead to more “play dates” in the future. And remember, recess is the best part of the day.

• Don’t pee in the sandbox, the pool or in any other communal area. Oh, wait, that’s another column for another time.

• A goodnight kiss is the key. Boys, take note: You did it on the playground in elementary school. Now, realize the value of something so elementary: the best way to get to a girl’s heart is through her lips. Especially at the end of a nice, long “play date.”

• And, finally: Stick to the rules of lunch-bag economics, and all transactions will run smoothly.

Remember afternoons in the cafeteria with your friends? Think: “I will trade you my Snickers bar for your bag of chips.” Basic supply-and-demand, give-and-take business affairs, which always resulted in the satisfaction of both parties.

It’s all about reciprocation — at least if you want to maintain good relations with members of the opposite sex.

To this day, I haven’t heard what happened to Mrs. Levy, but I realize I owe her a great deal of thanks.

Although I do not know if she is still around, I know that her wisdom is. It has done more than its share of persisting through the years.