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aussi - they were laughing because when he was a kid he was called the Face of Bo. The Face of Bo is a character who's been in a few episodes. Giant head in a glass Jar. He was in the start of this last series, where he died. But just before he told the Doctor that "you are not alone". Meaning there was another Time Lord out there. Which he knew about, because he was there in the 20th Century, still with a body, back when the Master was taking over.
Captain Jack can't die, and I'm guessing over the bajillions of years he gets so old he kinda mutates into a giant head.
I loved that twist. I giggled for about twenty minutes afterwards. Brilliant!!