ASK DR. BOLI.

Dear Dr. Boli: The hand sanitizer I just bought says it “kills 99.99% of germs.” But how do they know the germs are dead, rather than just quietly plotting revenge? That’s what I want to know. —Sincerely, A Reader with Very Clean Hands.

Dear Sir or Madam: Conveniently, germs, once they have gone to meet their maker, display a distinctive pattern of pigmentation in their microscopic eyeballs, known to microbiologists as the chiasmus, and resembling the English letter X. It is thus a simple matter for a scientist, or for that matter any hobbyist with a microscope, to determine whether any given germ has indeed kicked the bucket. Obviously, examining ten thousand germs to find the one still living is an unenviable drudgery, but there is nothing difficult about the principle involved.