What if i left the half drank but closed bottle of GrapeJuice on the Window Sill?

Question

And drank if 21 days later? Is that fatal? Is there a difference in how to ferment safely versus unsafely? i mean, hows this done?

Answer 1

you can't just take a bottle of grape juice and ferment it, fermentation involves yeast digesting and producing gas, you can buy the yeast online to make your own wine really cheap, all you need is a recipe that suits your taste

Answer 2

If the bottle has expanded then it has fermented. If not it probably just has a high bacterial load.  But the light has probably de-natured the flavinoids anyway (wine is normally  made in the dark) so bin it and buy some in.

Answer 3

Sounds like a moldy mess.  Why would you want to drink it?