This sounds like one of those questions that are meant to make
you squirm. Of course you don't know what other candidates have to
offer. You could always ask, but that's not recommended. The point
might be to see how you act when someone throws you a question that
makes you squirm.
So first of all, be cool. Keep your cool, be calm. It might even be okay to laugh. If I got asked this question, I would probably laugh and say "I have no idea."
But I wouldn't stop there.
It's also a great opportunity to mention anything that makes you unique. Here's a hint: look for a pair of skills or traits--a combination of two things that are special about you. Lots of people might have one ability or the other, but your combination of two might be unusual. Here are some examples:
- you're good at math AND you speak a second language
- you have nurses' training AND you're an amateur photographer
- you have an excellent memory AND you're very patient
- you are a good mechanic AND you know how to make people laugh