The sprogs raise an important scientific question.

I don't believe anyone has really defined the term "fart". It's an everyday language term, and probably necessarily at that level. Consequently, I agree that ANY sort of gas exchange qualifies as a fart, since it's a non-zero gas-exchange. However, there exists farts and FARTS and
F A R T S (and so on). So, an inaudible, odorless gas exchange qualifies as less of a fart than a stinky fat one, just like how Mikhail Gorbachev qualifies as less bald than Michael Jordan. I don't see this as a wrangling with distinctions. I see the question of "How much gas does it need to be to count as a fart?" as relevant once you say "No, I think there needs to be a macroscopic quantity of gas released all at once for it to rise to the level of a fart." since what does "marcoscopic quantity of gas" mean?

If we talked about "flatulence" with the Wikipedia definition "Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals." I'd believe that one could fairly easy make the case that it happens every second. Byproducts of our digestion come as certain microorganisms moving about and they no doubt expel a mixture of gases which go out our rectum as a result of the digestion at non-perceived levels frequently.

I'd classify the question in general as more or less half-philosophical and half-scientific.

As a mathematician/fluid-dynamicist I beg to disagree. It is clear that for sufficiently high volumes of fartgas released at sufficiently high pressure, the flow crosses a stability boundary which causes the anal sphincter to oscillate rapidly, emitting a characteristic "raspberry" noise. The stability boundary, the (if you will) fart catastrophe, represents the boundary between the true fart and mere gas leakage.

Some will, of course, object that this definition places the classic "silent but deadly" outside the definition of the true fart. I have two answers to this point
i) Anyone releasing a silent-but-deadly is already outside the pale of normal human decency so need not be taken into account, and
ii) So find a better definition, smart arse.

Now, if I can just work the above up into a little paper I should be well on my way to next year's IgNobel.

I don't have children, but I sent a link to a close friend who does. I got this answer [Note: Mouse is the (more) intelligent dog]:
Tara says there has to be a certain degree of pressure. Its doesn't have to make a sound because sbd.
Ollie says you have to feel it come out.
Rosie just said sbd and she is an expert.
Mouse says you have to try to avoid shocks or surprises if you don't want to get sent out of the house in disgrace.
We believe that sound-free and smell free is possible. Rosie testifies to this as she has done this and no one has noticed.
Tara thinks that people fart 10 times a day.
Rosie says the cheeks vibrate and that makes the noise.
As you can see it did generate a lengthy discussion.

I think it's a philosophical question. And maybe farting *isn't* a discrete property. Maybe it exists on a continuum, like "tallness." In that case, the sprog is more or less correct. Smart kid.

I remember, as high-school freshmen, discussing this with a friend of mine. He wanted to know what the energy potential was of an average fart--went so far as to talk it over with a Senior who happened to be in Physics at the time, although I suspect it had more to do with how attractive she was than the fact that she was in physics. He decided that the only way to reasonably trap said gases would be an inverted-container-in-a-bathtub method.

I understand (though did not see) that the Mythbusters came close to doing just that in one infamous episode.

I'd call it a good time to remind the sprogs that any operational definition is, by definition, imperfect, and that the multiple operational definitions used by different members of the scientific community are what allow us to converge on a "big picture" when each of us really only looks at a few tiles of the mosaic at best.

Sounds like they have their ScienceFair project for this year..

I recall, a couple of years back, reading of a scientific study of farting frequency. I think the first problem was defining 'normal". So this is a matter which more is known about than we realize.

I agree with csrster on this one. I am sure there is a bifurcation in the fluid dynamics eqns that can be used to define what a true fart is. Perhaps the silent ones represent a state where the viscous-elastic properties of the anal sphincter are abnormal? Yet, I still wonder how one can experimentally measure the volume of a "true fart". Have your kids thought about an appropriate experiment?

"Some will, of course, object that this definition places the classic "silent but deadly" outside the definition of the true fart. I have two answers to this point
i) Anyone releasing a silent-but-deadly is already outside the pale of normal human decency so need not be taken into account, and
ii) So find a better definition, smart arse."

With respect to answer i) one may merely respond that those who realize silent-but-deadly farts DO fall within the pale of normal human decency. Additionally, notions like "decency" don't really have a place within a scientific investigation. There exists nothing decent about applications of atom-splitting, but the Fat Man and Little Boy bombs still provide scientific confirmation of various hypotheses.

With respect to ii) there simply exists no need for a better definition. In fact, actually providing a definition has the potential to create an artifical precise boundary where at best there exists an imprecise boundary and perhaps none. Such artificial boundaries can cloud our perceptions and conceptions of how nature actually works. Such artificial boundaries often impede, for example, our understanding of evolution or perhaps phonemonena which quantum mechanics attempts to describe. So, a "better defintiion" may actualy make things worse.

Still, a viewpoint of farts as some sort of catastrohpic phenomenon may work. But, make no mistake it has to actually account for the 'silent-but-deadly' phenomenon instead of just hand-waving them off with talk of "indecency", nor can it can claim priority since it will have an inevitable crisp mathematical model (a "definition" in the sense of ii)) while the other has many possible fuzzy mathematical models, since we all do in fact smell, hear, and perceive farts differently to some degree.

I'm with Drug Monkey.

Aside from the obvious comedic value, kiddos (at the age to participate in a science fair) ought to be introduced to the semantic underpinnings of science, and why labels and class behavior are appropriate and important parts of the process.

Working Title: "Farting Classification System: Or, Why Pluto Is No Longer A Planet"

IMO, when setting the definition of "fart", you first need to specify whether you are considering the question from a scientific perspective, as distinct from judging it as an artform.

Anyone who has been "treated" to a competitive farting display by small to mid-sized boys should see where I'm coming from. And I understand that some continue to fart competitively even into adulthood.

Upon further reflection, it occurred to me that this might be a good time to have a discussion about the sense vs. meaning of a word with the dear little sprog.

I assumed that farts differed from other gas exchanges in that they are actively expelled from the anus, as opposed to gas exchange requiring no muscular or other participation. That would cover the "silent but deadly" variety, though without some way to measure muscle tension or activity in the emitters, it makes an operational definition from an observer's point of view harder. I assume that noise would imply muscle participation (sphincter release), though, so the presence of noise would be sufficient but not necessary.

I don't have anything to contribute to the main question, but I think someone should bring up Le Pétomane.

Schrödinger's Fart?