If you've ever secretly lusted after someone at work, you're not alone - one in five adults already in a relationship is in love with somebody else.
People usually fall for a colleague, close friend or an ex, says a study.
But it found people generally keep their desires under control - even though the average crush lasts three years and three months.
And about 16% of those who admit they love another person will become involved in a long-term affair, with men being far more likely to have their heads turned.
And 29% of men in an affair said they planned to leave a partner for the other women compared to 19% of women.
www.OnePoll.com, which carried out the study of 3,000 people, said: "Men have a bad reputation when it comes to eyeing up other women yet the research proves this is true.
"Men are much more likely to 'window shop' whilst in a loving relationship as well as falling for another women.
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"Even in a happy relationship, it's seems to be possible to have a wandering eye or even crave affection from another person."
But better news for the long-suffering partners of those with a wandering eye is that falling for someone else usually only happens once during each relationship.
And only 6% said they were planning to leave their long-term partner for the other man or woman.
Ex-boyfriends or girlfriends also feature highly on the list of potentially dangerous love interests.
And half of those 'completely happy' with partners have had strong feelings for at least one other person.