Are Certain Questions Never Safe to Ask?

I was at my daughter's dentist yesterday, where a heroically patient hygienist was teaching my two-and-a-half year-old how to brush and floss her teeth. Marveling at the way this woman sweetly but firmly dealt with my toddler, I asked, "Do you have any kids of your own?" As the words were coming out of my mouth, I got a sinking feeling that I shouldn't have asked, and her response confirmed that sixth sense.            

She told me she'd lost twin boys when she was five months pregnant, and...