Source: Did You Know: The Lines On A Solo Cup Are Measurement Marks | KUNC
Stashed in: Alcohol!, Beer!, snopes!
I didn't know this...
Apparently it's just a coincidence that the lines correspond to measurements.
This explains: http://www.snopes.com/food/prepare/solocups.asp
I didn't know this, either.
I also liked the link on how to build a campfire: http://kunc.org/post/summer-science-how-build-campfire
I saw another, more crude variation on this theme today.