There's a man selling bak chor mee at the shopping mall near my office who flirts with me and I with him.

He's the friendly, jokey sort and always has a nice smile for me. Because he doesn't give off lecherous vibes, I always smile at him too.

Once, when I said I wanted to da bao ('pack' or 'take away' in Mandarin) an order, he smiled slyly and muttered 'da bao wo' - 'pack me', literally.

It cracked me up and I laughed out loud - men appreciate it when women laugh at their jokes, don't they? And this was a genuinely funny chat-up line too.

It's harmless flirting and makes for a bit of silliness. We aren't in the least bit attracted to each other and I'm sure he uses that line on other female customers as a sales ploy. Still, I was flattered by that playful remark. It brightened my day.

Flirtation on this level is just a bit of fun. You would have to be really insecure or a killjoy to be offended by an exchange like this.

Anthropologists say that flirting is a basic human instinct. It is part of how people interact and it happens in all cultures.

Flirting has a biological purpose too. If we didn't express an interest in the opposite sex - which is essentially what flirting is about - we wouldn't go on to reproduce and the human species would die.

Once upon a time, the art of flirting was relatively straightforward.

You spot someone you like and if he or she is also interested in you, you make eye contact.

If you are a female, you might flick your hair or touch your face to indicate further interest. If you are a male, you might lick your lips, puff up your chest or flex your attractively muscled arm.

You might go on to start a conversation. He or she might tell a joke and you would laugh at it appreciatively. You give verbal cues to encourage each other to go on talking. You might even lightly brush his or her arm. If the chemistry is really there, a deeper connection is forged.

That's the traditional form of flirting anyway. But with the rise of the Internet, mobile phones and social networking sites such as Facebook, flirting in the 21st century has become a lot more complicated.

People who would never dream of flirting at a party or a bar or at work are now doing so under the cover of a phone or computer.

Technology has made flirting simple, fast, intimate and private.

Where once you had to muster the courage to go up to a person you fancy, strike up a conversation and hope he or she is keen on you too, you can now text or e-mail 'Hey, you look good today' in 15 seconds flat and send off that flirtatious message.

If the person doesn't respond, it's not that big a deal because you aren't rejected to your face. In fact, you can even brush off the whole thing by convincing yourself that your remark wasn't even flirtatious in the first place but merely friendly.

The problem is if you are already in a relationship. Cyberflirting with someone other than your partner can cross the line into something illicit.

When you e-mail or SMS a person, the exchange is more often than not between just the two of you. Even if the message is not flirtatious, privacy can lead to intimacy and secrecy.

And because you don't have to look the object of your flirtatious attempts in the eye, you tend to be more daring and your messages could contain saucy subtexts.

It's natural for us to seek some attention from the opposite sex but there's a line between a bit of harmless flirting and being disloyal to your partner.

We saw an example of this in the recent murder trial of Singaporean Ram Puneet Tiwary in Sydney, who's accused of killing two Singaporeans there in 2003.

His defence lawyer revealed a trail of flirtatious Internet chats which one of the victims - who was married - had with a former schoolmate. The man spoke about being jealous of her boyfriend, of seeing a photo of her in a bikini and of being crazy about her.

Maybe it was all said in jest, but where does one draw the line? If you make flirty remarks to a person who isn't your partner but never act on them, is that cheating?

Is even the mere act of sending such messages already disloyal? What sort of private space do people who are attached have, to do and say what's really on their mind to a person of the opposite sex?

Say you are a married mother who stumbles upon a major crush from your school days through Facebook.

He has been at the back of your mind all these years and you now have the opportunity to contact him and say 'hello'. Is that flirting? Is that being unfaithful to your husband?

Probably not if the mail is written such that it is something you would also send a former classmate who's the same sex as you. But what if in your mail you reminisce about how 'exciting' and 'wonderful' those good old days with him were, how you miss them and want to relive them, and how you have been thinking of him? Does this cross the line?

Probably, for it implies you not only still have feelings for him but also want to act on them.

What if your ex-flame writes back and you take the next step of meeting up for lunch? Just the two of you. Does that constitute flirting and infidelity?

The problem is that our motives aren't always straightforward or pure.

You might want to know what's been happening to an old school crush, but are curiosity and platonic friendship your only motives, honestly?

Are you actually yearning for something more to be rekindled? Or testing the waters to see if you could still 'attract' that person, just to stroke your ego and vanity or to appease an insecurity of some sort?

Behavioural experts say there are several signs you should watch out for to tell if flirting is heading towards dangerous waters.

First is if you've become too emotionally involved. If you find yourself feeling upset that the object of your message does not reply at once, or find yourself on a high when he does, alarm bells should be ringing. The person has become too important to you.

Another gauge is how open you are about the object of your flirtation to your partner.

If you keep your partner in the loop about your correspondence, even if it is flirtatious, then you're on safe ground. But if you find yourself covering your tracks or toning down what has been happening, think again.

Experts say another gauge is to imagine what it would be like if the tables were turned. How would you feel if your partner was doing what you did? If you'd be fine with it, you don't have a problem. But if you'd be unhappy, your actions are potentially hurtful.

Life is complex. People are complex. Relationships are complex. Ultimately we are all searching for love and approval, aren't we? And technology is aiding us in this by making flirting so easy.

It's up to us where we want to draw the line.

As far as the bak chor mee man is concerned, I say bring the flirting on. It's funny and makes me happy. Da bao wo? Any time.

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This article was first published in The Sunday Times.