In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: When Lucy and Maria Aylmer tell people they are twins, disbelief is the response. The 18-year-olds from Gloucester, U.K. are two of the five children born to their Caucasian father and "half-Jamaican" mother. While their other siblings (brothers and sisters) have a blend of features from their parents who have different skin colours, Lucy and Maria are unique: Lucy has fair skin and red hair, while Maria has brown skin and dark hair. "No one ever believes we are twins because I am white and Maria is black," Lucy said. "Even when we dress alike, we still don't look like sisters, let alone twins. When we meet friends for the first time they all refuse to believe we are twins and some have even made us produce our birth certificates to prove that we are really twins." Unlike identical twins, fraternal twins are bom from two separate eggs. The BBC reports that for a biracial couple expecting twins, there is about a 1 in 500 chance those twins will have different skin colors. The Aylmers are proud of their uniqueness. "Now we have grown older, even though we still look so different, the bond between us is much stronger," Lucy said. "Now we are proud of the fact that we are each other's twin sister. Maria loves telling people at college that she has a white twin sister -- and I'm very proud of having a black twin sister, too."
Answer: they are twins with different skin colours
Lucy and Maria are unique because _ .