Christmas is the busy season for ham sniffers

> I read an article in Science about bloodhounds, and how they could smell so very well. The authors described the various experiments that they did--the bloodhounds could identify which items had been touched by people, and so on--and I began to think: Itis very remarkable how good bloodhounds are at smelling, being able to follow trails of people, and so forth, but how good are we, actually?

> When the time came that I could visit my wife, I went to see her, and I said, "We're gonna do an experiment. Those Coke bottles overthere (she had a six-pack of empty Coke bottles that she was saving to send out) --now you haven't touched them in a couple of days, right?"

> "That's right."

> I took the six-pack over to her without touching the bottles, and said, "OK. Now I'll go out, and you take out one of the bottles, handle it for about two minutes, and then put it back. Then I'll come in, and try to tell which bottle it was."

> So I went out, and she took out one of the bottles and handled it for quite a while--lots of time, because I'm no bloodhound! According to the article, they could tell if you just touched it.

> Then I came back, and it was absolutely obvious! I didn't even have to smell the damn thing, because, of course, the temperature was different. And it was also obvious from the smell. As soon as you put it up near your face, you could smell it was dampish and warmer. So that experiment didn't work because it was too obvious.

> Then I looked at the bookshelf and said, "Those books you haven't looked at for a while, right? This time, when I go out, take one book off the shelf, and just open it --that's all--and close it again; then put it back."

> So I went out again, she took a book, opened it and closed it, and put it back. I came in --and nothing to it! It was easy. You just smell the books. It's hard to explain, because we're not used to saying things about it. You put each book up to your nose and sniff a few times, and you can tell. It's very different. A book that's been standing there a while has a dry uninteresting kind of smell. But when a hand has touched it, there's a dampness and a smell that's very distinct.

> We did a few more experiments, and I discovered that while bloodhounds are indeed quite capable, humans are not as incapable as they think they are: it's just that they carry their nose so high off the ground!

> (I've noticed that my dog can correctly tell which way I've gone in the house, especially if I'm barefoot, by smelling my footprints. So I tried to do that: I crawled around the rug on my hands and knees, sniffing, to see if I could tell the difference between where I walked and where I didn't, and I found it impossible. So the dog is much better than I am.)

From Surely You're Joking Mr. Feynman