Now you see it, now you don't. That's the logic behind the vanishing spray being used at this year's World Cup in Brazil.
If you watched Thursday's opening Brazil-Croatia match, you may have seen it: Before a free kick in the first half, a referee pulled a spray can out of a holster and squirted a white line on the field.
No, the World Cup refs aren't wannabe graffiti artists. For the first time, FIFA referees are using the special spray to paint a temporary white line 10 yards from the free kick spot, marking the safe area into which opposing players cannot encroach.
The spray disappears after one minute.
This, although somewhat awkward to watch, addresses a problem for referees and players alike: Although they're supposed to remain 10 yards away, opposing players have frequently crept closer to the free-kick taker in an effort to disrupt the kick, starting countless arguments about fairness.
The vanishing spray, which was an immediate hit Thursday on Twitter, is the brainchild of Argentinian journalist and entrepreneur Pablo Silva.
"It started seven or eight years ago when I was playing in a championship played amongst former school members," Silva told Reuters.
"In the 88th minute, we were losing 1-0 and won a free kick on the edge of the area. When I took the kick, the wall (of opposing players) was three meters away. The referee didn't book anyone and didn't do anything. We lost the game and driving home later, with a mixture of anger and bitterness, I thought that we must invent something to stop this."
The spray is called 9:15 Fairplay, or 10 yards expressed in meters. It's already been used in games the past several years by Major League Soccer.
"We find the vanishing spray to be extremely useful and very effective in ensuring the defenders are 10 yards from the ball," said Paul Rejer, training and development manager for the Professional Referees Organization, which supplies officials for MLS.
"The spray makes it clear where the ball is to be placed and where the defenders have to stand. Since the use of spray we experience very few problems in achieving the ... (minimum) distance."
Rejer said the spray has been welcomed by officials and is no hassle for them to carry during games.
If nothing else, Silva and his spray already have some high-profile supporters, including Portuguese soccer superstar Cristiano Ronaldo.
"I think it's a great idea," he said in a testimonial on the 9:15 website. "Use a spray to mark the distance between the barrier and the kicker can be a great help."
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