What colour is this tennis ball? Photo / Getty Images
It is a question that should have a simple answer but it has caused a massive debate online as people just can't seem to agree on what colour tennis balls are: yellow or green?
The question has caused so much discussion that even the world's top male tennis player has weighed in.
In a video that has been viewed nearly 370,000 times, the question of the true colour of tennis balls was posed to Roger Federer.
"They're yellow, right?" he said, with slight uncertainty.
He then repeated more confidently, "They're yellow!"
For some people Federer's answer was enough to settle the debate.
"I would have said green but I think we have to all agree on yellow now," one Twitter user wrote.
Another said: "What Roger says must go"
But for others, the tennis pro's answer did nothing to change their mind.
"Tennis balls are green. Why is this a question. Roger Federer is wrong," another person said.
"How can you play tennis all your life and think tennis balls are yellow," one person wrote.
But according to the International Tennis Federation only white and yellow balls are approved, with fluorescent yellow, also known as "optic yellow" being most commonly used.
So there you have it!
New York Times: Experts say the answer is a puzzle.