Some background info (that the team mostly doesn't have): Jack Harkness (not his real name, obviously) came from a human colony world in the 51st Century. When the viewers first met him (in Doctor Who, with Chris Eccleston), Jack had informally retired from the Time Agency, when he learned they had retconned away two years of his life (and now you know as much as we do - and as much as Jack does - about that). He was making a little money on the side, running a scam in WWII London (a popular place/time for temporal scam artists, as so much of it was destroyed in the war, and they know which spots). He became a Companion for a short time, along with Rose, until they encountered the Emperor Dalek aboard the GameStation about 200,000 years from now. The Emperor almost killed the Doctor (who had sent Rose away in the TARDIS), and did kill Jack. Rose, however, in order to save the Doctor, absorbed/joined with the Heart of the TARDIS, giving her the power of a god. She returned to the year 200,000, eliminated the Daleks (aside from four who executed an emergency temporal shift), and resurrected Jack, before the Doctor returned the power back to the TARDIS (and triggered his next regeneration, into David Tennant). Sadly, Rose didn't have much experience at resurrection, and fixed Jack's status to the point just before he'd been killed. As a consequence, no matter what happens now, he's stuck at that point - he is a temporal anomaly. This caused the Doctor to flee in fear. Jack was able to use his own temporal device to jump back toward the one era he was sure he'd be able to find the Doctor in - he was trying for WWII Earth - but the device was breaking down, and deposited him on Earth in the late 19th Century instead.
What the team does know is that he's been around for a long, long time - he's told them of things that happened to him during WWI - and that when he gets killed, he comes back. They've figured out some things about time travel, especially since his old partner, Captain John, came by in the Series 2 opener, and they know that he has a child (that he had borne). And they know that his sexual preference is "yes" (as the Doctor described it to Rose when they first met Jack, "So many species, so little time").
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.