In Michigan? Tell them it's like cross country skiing, just with different equipment. Here in Minnesota, I've seen the light bulb click on more than once with that comment.

And interestingly enough, it was a co-worker that got me into winter commuting. After a 6" Nov snowfall in '94, I'd parked the bike for the winter, only to hear a co-worker the next morning jokingly chide me for wimping out. I showed him the next morning and rode in, and haven't stopped. :-)