This discussion may contain spoilers!
While you are searching for supplies, you can knock on a window to get Alvin's attention. You can then convince him to help you by getting you some supplies, and if you do, he will get the bandage that you can find on your own, and a juice box!
When Clementine stitches her arm, you can then drink the juice box, if you got Alvin to give it to you. You can drink it until it is empty.
Perhaps Clementine saw the juice box in the preview! She had no reason to believe the juice box was alive, so her reaction makes sense. It would also be such a plot twist! The juice box was never attacked by the zombie, so it's fine. The juice box must be what Clementine saw! J00S CONFIRMED!