f(x)'s member Krystal revealed why she makes a straight face all the time.

In MBC entertainment program "Radio Star" aired yesterday, Krystal said, "I don't really like my smile," revealing the reason behind her being called an 'ice princess.'

Krystal also revealed that Girls' Generation members watch rated-R adult movies in their dorm. She said, "I once went to their dorm and they were watching the movie 'American Pie.' There was a sexual scene so I went inside but they dragged me back out and made me watch it."

Photo Credit: OSEN