DIY CO2 generator

[Bk718]

I'm curious, what does the gatorade bottle do?

its a gas separator/bubble counter

 

[JuanitoW]

Which would lead a n00b like myself to ask what is a gas separator.
The answer lies here:
http://www.qsl.net/w2wdx/aquaria/diyco2.html

it separates the gas from the yeast liquid which can clog up the pipe.