Why does water go sour?

Question

Ok,

picture the following scenario:

I have 1 clean bottle,

fill it up with water,

make sure the cap (?) is firmly secured,

then leave the bottle on my desk for 1-2 days,

and when I come back,

it smells like old socks.

Anyone got an explanation as to why water left a couple of days smells like this?

Answer 1

youre not supposed to do that because the water and bottle interact and its carcinogenic.

Answer 2

Use a glass bottle and not a plastic one.