its because to make it, you "butterfly" a gallon milk jug, much like you would butterfly a piece of meat!
Ill post instructions and pics later.
--EDIT--
Here are some pics.
And the instructions are pretty simple. All you need are maybe a knife, scissors, and plastic milk jug, gallon size.
1) Cut off the end where the cap goes, down to where you can cut around it and get it off
2) Cut down from opposite side of the handle, until you hit the thick part of the bottom.
3) Use the knife to cut through the thicker part.
4) Continue with the scissors until you hit the handle.
5) Do you see the flaps you have made? Turn them inside out.
6) After you do that, you will have two sides and a handle.
You can customise, or paint it or whatever, it's open from there.