I’m always intrigued by the juncture of dissimilar materials. Take water and oil, for example. Initially they refuse any contact, but if you put both in a jar and shake vigorously, they’ll quickly amalgamate into a far more friendly blended material.

Or consider the circular knitting needle. Getting a perfect transition from solid needle to flexible nylon cord is the bane of every needle manufacturer’s existence. Done poorly, your yarn will always stall on its way across. Done well, the transition is seamless.