Are these tennis balls yellow or green?
這些網球是黃色還是綠色的?
It seems a simple question - the tennis balls are obviously yellow. But some people say they are obviously green.
這似乎是一個簡單的問題 - 網球顯然是黃色的。但有些人說顯然是綠色的。
And then there are the people who say they are obviously both.
然後有人說顯然是兩者兼有。
So what colour are tennis balls? Fortunately, the ATP world number one has provided us with an answer.
那麼網球是什麼顏色?幸運的是，職業網球聯合會世界頭號人物為我們提供了一個答案。
In a video viewed almost 315,000 times at the time of writing, tennis star Roger Federer is asked what colour tennis balls are.
在發布後獲得315,000次觀看的視頻顯示，網球明星羅傑·費德勒被問及網球的顏色。
"They’re yellow, right?" says the Swiss.
「是黃色的，對吧?」瑞士籍的費德勒說。
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".
對於一些人來說，費德勒的話就是標準答案，有一個網友說，「如果羅傑說是黃色，就是黃色」，另一個網友說，「我一直以為網球是綠色的，但如果羅傑·費德勒說它們是黃色的，我不會爭辯「。
Despite one person’s protests that if "the grass at Wimbledon is green, the balls cannot also be green", others remained unconvinced.
儘管有人發出爭議說，如果「溫布爾登的草是綠色的，網球不可能是綠色的」，但這沒有說服其他人一些人。
"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.
「羅傑·費德勒是錯的」，一位推特用戶宣稱，另一位則表示它們是一種「螢光黃色」，也可以被視為綠色。
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?
那麼為什麼有些人看到不同的顏色?答案也許可以在之前網絡上的人們對一件白金色的裙子的錯覺中找到，或者，它是藍黑色的?
The photo of a dress divided people on social media, with #TheDress trending worldwide in February 2015.
在社交媒體上，人們對這件連衣裙的照片有分歧，導致2015年2月的話題#TheDress在全球範圍內成為熱門。
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.
當時利茲大學色彩科學和技術主任史蒂芬韋-斯特蘭教授說，人們看到色彩的方式差異很大。
"We don’t always see colour in the same way," he said. "The surprising thing is that this doesn’t happen more often.
「我們並不總是以同樣的方式看到顏色，」他說，「令人驚訝的是，這並不是經常發生。」
"It is possible that people could literally be seeing different colours but it’s impossible to know what is in someone’s head."
「人們可能真的會看到不同的顏色，但我們不可能知道它在某人腦中是什麼顏色。」