A couple of weeks ago, I was playing with a vanilla guy who turned out to have a very ticklish body. I immediately set about nibbling everywhere and tickle torturing him, which got him practically into hysterics. His face was buried in his pillow trying to smother the shrieks he was making. Suddenly he shouted "Periwinkle!" Somewhat confused, I stopped and said "Is that a safe word?" and he said "I think so. It's the only thing I could come up with."