Styrofoam Cups

I drink coffee a lot regularly. When I drink coffee from a Styrofoam cup with a lid, I really feel that I am living in the U.S. I especially like ordinary Styrofoam coffee cups with lids that have a removal tab on the rim. Most donut shops use these kinds of cups. These don’t exist in Japan. There are only a few types of disposable cups with lids in Japan. I think it is because “To go” has become popular only quite recently in Japan. I don’t think coffee cups for takeout were popular before Starbucks launched their business in Japan.

One day, I bought coffee at a donut shop and brought it back to the office. The coffee was in a Styrofoam cup. I put the cup on the desk. After a while, I saw there was coffee on the desk. I thought I had spilled it accidentally, so I wiped my desk and cup. A while later, I found spilled coffee again and wiped my desk and cup again. After that, I briefly glanced at the cup, and I noticed there were small brown spots on the cup. I looked at the cup carefully, and I was surprised. Coffee was oozing from the polystyrene cup!!

I have no idea what could have caused this phenomenon. I would at least like to know whether the coffee or the cup were factors. I took a picture (See below). If you have any idea why this happened, please let me know.