One Important Thing: Don’t Taste It
If you don’t know what was added to the bottle, don’t drink it or taste it to figure out what it is.
The safest approach is to ask your niece what the experiment is and what ingredients she used.
If it is an active fermentation, the container can also produce gas and pressure, so it shouldn’t be tightly sealed without an appropriate way for gas to escape.
What About the Giant Balloon?
That’s actually consistent with the basic idea.
As carbon dioxide accumulates, the balloon can inflate. Educational demonstrations use this exact principle to make otherwise invisible gas production visible.
So the strange-looking setup isn’t necessarily anything mysterious.
The simplest explanation:
The balloon is probably there to capture or indicate gas produced inside the bottle—most likely as part of a fermentation experiment.
Whether it’s a harmless science project or something intended to produce an alcoholic beverage, however, the photo alone can’t establish that.
If I found this in a teenager’s closet, I’d be less concerned about the balloon itself and more interested in simply asking:
