It seems a simple question - the tennis balls are obviously yellow. But some people say they are obviously green.
这似乎是一个简单的问题 - 网球显然是黄色的。但有些人说显然是绿色的。
And then there are the people who say they are obviously both.
然后有人说显然是两者兼有。
So what colour are tennis balls? Fortunately, the ATP world number one has provided us with an answer.
那么网球是什么颜色？幸运的是，职业网球联合会世界头号人物为我们提供了一个答案。
In a video viewed almost 315,000 times at the time of writing, tennis star Roger Federer is asked what colour tennis balls are.
在发布后获得315,000次观看的视频显示，网球明星罗杰•费德勒被问及网球的颜色。
"They’re yellow, right?" says the Swiss.
“是黄色的，对吧？”瑞士籍的费德勒说。
For some people Federer’s word was enough, with one person suggesting "if Roger says they’re yellow, they’re yellow", and another saying "I’ve always thought tennis balls were green, but if Roger Federer says they are yellow who am I to argue".
Despite one person’s protests that if "the grass at Wimbledon is green, the balls cannot also be green", others remained unconvinced.
尽管有人发出争议说，如果“温布尔登的草是绿色的，网球不可能是绿色的”，但这没有说服其他人一些人。
"Roger Federer is wrong" declared one Twitter user, while another suggested that they are a type of "fluorescent yellow" that can also be seen as green.
“罗杰•费德勒是错的”，一位推特用户宣称，另一位则表示它们是一种“荧光黄色”，也可以被视为绿色。
So why do some people see different colours? The answer may lie in a previous optical illusion that made waves online about that white and gold dress. Or was it blue and black?
At the time Professor Stephen Westland, chair of colour science and technology at the University of Leeds, said the way people see colours varies hugely.
当时利兹大学色彩科学和技术主任斯蒂芬韦-斯特兰教授说，人们看到色彩的方式差异很大。
"We don’t always see colour in the same way," he said. "The surprising thing is that this doesn’t happen more often.
“我们并不总是以同样的方式看到颜色，”他说，“令人惊讶的是，这并不是经常发生。”
"It is possible that people could literally be seeing different colours but it’s impossible to know what is in someone’s head."
