A proclamation of sexual attraction. A hand resting on the knee. A flirty text message.
From the right person at the right time, they can make you feel great.
But from the wrong person or at the wrong time, an innuendo-laden text becomes creepy and an unwanted touch can make you feel uncomfortable and ashamed.
As the number of women making claims against Hollywood film producer Harvey Weinstein grows by the day, women around the world have spoken on social media about their experiences of sexual harassment under the #metoo Twitter hashtag.
Weinstein wielded great power, able to make or break his alleged victims' careers, but harassment can be just as damaging away from work.
In a global debate, the question of how we define sexual harassment is not altogether clear.
And that line between flirtation and harassment is a very fine - and often blurred - one.
So how do you ensure you stay on the right side of it?
If you want to meet someone, you have to flirt, says relationship expert James Preece.
But it's about doing it in the right environment, not when people are least expecting it, he says.
The problem is men can't always read the signals and assume all women are interested in them, while women can be huggy and tactile, and they'll say they're just being friendly, he says.
He advises his clients - men and women aged from 23 to 72 - to play it safe by flirting in a playful - not a sexual - way.
"Treat them like your mother at the first meeting," he says. "Be friendly and build up a rapport and trust."
At the end of the first date, he suggests a friendly hug or peck on the cheek.
If you get a second date, try touching them on a non-sexual body part - such as below the elbow or towards the small of the back, he says.
If they don't flinch, you can go in for the kiss.
When does flirting become sexual harassment?
When it's unwanted and persistent, says Sarah King, of Stuart Miller Solicitors.
Dating expert James believes it's when a man pushes things too far - whether through what he says or what he does - when a woman clearly doesn't want it.
Sea Ming Pak, who goes into London schools to teach young people about sex and relationships, reels off a long list of what she thinks constitutes sexual harassment: non-consensual touching; feeling entitled to someone else; talking in a certain way; chasing girls down the street in order to chat them up; wolf-whistling and using a position of power or trust to talk in a creepy way.