Arguably the best of John Carpenter's horror movies after Halloween, The Thing tells the story of a shapeshifting alien parasite stalking an American research station in the Antarctic and taking the forms of its victims. After Kurt Russell's R.J. MacReady loses all of his friends and ultimately defeats the last morphing alien, he's left in a weary standoff against the only other survivor, Childs (played by Keith David). The movie ends ambiguously, with both men leery of one another, likely freezing in the subzero temperatures as the compound's fires start to die. Even worse, we're left unsure whether MacReady or Childs is a converted alien or human. Fortunately, multiple viewings and a mindful eye help narrow down our choices.
When Mac and fellow survivors Nauls and Garry left the compound to search for their incarcerated scientist, Blaire (who was unknowingly set free and possessed by the Thing), Childs was left at the compound's main door keeping a lookout. We find out that Blaire/Thing was in the generator room the whole time, which was really close to where Childs was keeping lookout. Since Childs was watching from the window to see Mac and company go, Blaire could've easily sneaked up on him from behind. More importantly, the dark blue jacket Childs wore when he was keeping lookout changed to a beige/brown by the end of the movie (like Kubrick, Carpenter was heavy into details), which suggests the alien tore through his coat and he took another one from the extras by the exit door.
Even more important is the exchange between Childs and MacReady at the end. During this scene, Kurt Russell was clearly wearing a flamethrower on his back, covered by a cloak, though it's never mentioned. He's also seen carrying a bottle, which he doesn't drink but shares with Childs, who was the most distrusting member of the group and normally wouldn't touch it. Mac was a heavy drinker and always boozing whenever he had that bottle with him, but he and other members of the team filled all the liquor bottles with gasoline to destroy the compound, which could suggest why he wasn't drinking from it.
Carpenter himself has said that Mac's breaths are visible, whereas Childs' aren't, and he's also pointed out the "gleam" that's always present in the true humans' eyes in the film. Referring back to the chess game at the beginning of the movie, where Mac loses a match against a computer (an artificial "Thing," standing in for a human), he ultimately gets the last laugh by pouring alcohol into the computer, shorting it out. Here, we can imagine Mac getting the last laugh in a similar manner: He lost the game (all of his friends are dead, the compound is destroyed, and he'll inevitably die out there), but gets the last laugh by giving Childs (presumably the Thing standing in for a human) a drink from what is likely a gasoline-filled bottle, before blowing him away with the hidden flamethrower. Of course, the Thing wouldn't really know what whiskey is supposed to taste like, right?