When multiple people drink from the same container, there will usually be some amount of backwash put back into the container; as the number of sips increases, in general, the concentration of backwash also increases.
Also a good way to determine if someone has eaten fish or tomatoes recently.
Tat: Yeah, here you go.
Soph: Is that backwash at the bottom?
Soph: You keep it.
(incidentally, if anyone gets a bottle with backwash in the bottom, try refilling it with its original liquid as well, so it's full, then heating the bottle from below. Because it's seperate, I wonder if it behaves like a lava lamp. If anyone wants to try this, feel free to enlighten me.)
Sven- "Just some backwash of water chestnut and sesame"
William- "Why on gods green earth were you eating water chestnuts and sesame seeds?"
Sven- "I.. dont.. know..."