Definition: 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.
Input: Article: Probably many of you know the idiom "an eye for an eye and a tooth for a tooth". But do you believe a tooth for an eye? That's something a little bit different. This strange exchange is neither a form ofretribution  nor an unexpected gift from the Tooth Fairy --- it's actually a very generous gift from a young Irishman, Robert McNichol, 23, to his formerly blind father, 57-year-old Bob McNichol. In 2005, Bob McNichol's sight was destroyed in an accident when his eyes weredoused(......)with liquidaluminum . Doctors predicted that he would never be able to see again. But McNichol never liked to take no for an answer: when he heard about a unique operation called Osteo-Odonto-Keratoprosthesis, he knew he had to _ , even though the success rate was a mere 65 percent. In the technique, the eyesocket  is rebuilt and implanted  with a human tooth and part of the jawbone, to provide support for an artificialcornea . Luckily for McNichol, Robert was willing to sacrifice one of his teeth to help his father regain his sight. The technique isn't simple: McNichol's first stint on the operating table lasted ten hours, with a five hour follow-up session at a later date. But thanks to his son's gift, he's already seeing astonishing results. "Now I have enough sight for me to get around and I can watch television. I have come out from complete darkness." he told his friends. 
 Answer: because of an accident
