Picture yourself having a good time on a date with someone you are interested in. The person’s phone rings and as you watch recognition of the number roll over their face like the haze of a fog-filled Isla Vista morning, you know you have a power struggle ahead of you. If the next thing you experience is the chime of their voice saying, “Sorry, I have to take this really quickly,” you know it can’t be good.

I have a friend who turns into emotional putty when his ex-girlfriend calls. He can be partying, having a great time, and then she somehow gets involved and we have lost him for the night. I don’t know what she blabs about to him every time she calls, but from what I have observed, she could even say, “Hey babe, I’ve contracted leprosy!” and he would rush right over to see her.

So what is the mysterious power the ex has over us? It seems that, especially if the situation ended badly, there should be no intrigue for the ex-factor, because the point is, the relationship did end. But since liaisons cease for such a wide array of reasons, it turns out frequently there is still unfinished business to take care of between two people who did not work out as a couple. Even just the curiosity of experiencing a hookup with that person now that you can’t have them may be incentive enough to blur the lines between old friends and new fucks.

Sleeping with an ex sometimes means trading in your ride on the semi-stable friendship train for being dumped off at the serious emotional baggage claim. One moment you appear to be just normal friends who used to date, and the next you are ripping each other’s clothes off with such anticipation and awkwardness, it’s like watching your very first porno.

After the fact, if you feel confused, you are probably not the only one. A spur of the moment ex-capade is no easy thing to analyze. Truth is, motivation for dipping into the ex-files is usually pretty simple. It took so long to break them in and you still love them despite their flaws, which makes them almost too comfortable to give up. Plus, I bet you are pretty sure that your trusty old Jack Purcells don’t have any strange, inexplicable venereal diseases.

It is easy for exes to win attention in competition with a newcomer because they are more easily attained. If the two exes are up to getting down, they can do so free of all of that getting to know you and impress you, I am not sure if you dig me bullshit. Convenience is key.

However if your ex dumped you by saying, “Having sex with you was worse than being beaten with split bamboo, and even then it topped our conversations!”, chances are there is no ex sex with that person in the future for you. But if the two of you decide that you wouldn’t mind exchanging some fluids for old times’ sake, it’s an easy well in which to fall.

Please, people, never take for granted the fact that there was a reason the relationship happened and there were perhaps several why it ended. So don’t delude yourself into believing that there won’t be at least some emotional ramifications on one side or the other when the lust ends and the reality sets back in. And if you still do care about that person, maybe consider giving it another go with them. Who knows what new tricks they may have up their sleeve? When all is said and done, though, and the curtain falls on your platonic relationship to allow for some backstage nookie, remember: Sex with an ex is never simply sex.

Kate Rice is the Daily Nexus sex columnist.