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.