I got my first job at 15 answering this question. I sold cordless (not cellular, cordless) phones at Sears
The "easy" answer to this question is to pick up the pen, then ask the interviewer if they can write something down. They'll look for a pen, which you're holding. You smile, and say, "if you'd like, I have something to assist with that request..." and trail off a bit. Some people will laugh at this joke and it's enough for them to pass the test. Personally, I'm not a fan of this method.
My 15 year old self hadn't heard this question before, so I just picked the pen up and started listing the wonderful qualities of this pen, and how quickly your life would be enhanced if you had it. "It has the deepest of royal blue hues, with a rich writing sensation and smooth flow on the paper. The grip allows for ultimate comfort, so the pen is usable all day long without any soreness in the fingers or hand. It's so well shaped and ergonomical, you can barely feel it in your hand. And with the above-industry-standard sized ink chamber, you'll be able to use this pen far longer than the competition. This pen has both the value and form that the modern consumer has come to expect in their premium pen buying experience."
And so forth. Basically, do sales stuff
Keep business and family separate.