A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How does one suck in a piece of spaghetti?

February 14, 1997

Dear Cecil:

How does one suck in a piece of spaghetti? Think about it. How one sucks milk through a straw is easy. The lowered pressure in the mouth due to sucking causes the air pressure over the milk to force the liquid up. But if one pushes on the end of a piece of spaghetti it just buckles. The mouth is closed and sealed over the sides of the spaghetti, so passing air doesn't drag it along. Somehow the air very close to the mouth must obliquely communicate a force along the length, and it's far from clear how it's possible.

Cecil replies:

You're thinking, this is the lamest question Cecil has ever answered. However, this is because you lack an appreciation of the scientific issues. I blame myself.

It took me a while to get the regulars on the Usenet newsgroup sci.physics to focus on this too. Apart from the one lamer who said the partial vacuum inside your mouth exerted a positive force that pulled the spaghetti in, most reasoned as follows:

  1. Air pressure is customarily conceived of as acting perpendicularly to the surface on which it bears. In other words, it presses straight down.
  2. Air pressure at any point on the side of a strand of spaghetti is exactly counteracted by the air pressure on the opposite side.
  3. The one place where the air pressure is not counteracted is on the end of the spaghetti. The pressure on the outside end is much greater than the pressure on the inside (mouth) end.
  4. Therefore, the force on the spaghetti is equal to outside air pressure minus the pressure inside your mouth times the cross section of the spaghetti.

You're not getting this, I said. I know how much pressure is exerted. What I want to know is where it's exerted, since it seems pretty obvious that literally pressing on the end of a strand of limp spaghetti doesn't do jack.

What do we care where it's exerted? said the sci.physics regulars. We are scientists. We deal in the world of quantifiable effects. It is enough to know that the air bears somewhere, and that the pressure differential in aggregate is some mathematically determinable amount, as a consequence whereof the spaghetti is sucked, or rather forced, into your mouth.

Freaking gearheads, I said. Screw the mathematics. I want to know, what is actually happening at the level of individual particles?

"Heisenberg tells us …" the sci.physics types began.

Screw Heisenberg, I replied.

Finally a few of the scientific types conceded that the question had a certain practical interest. After some discussion we concluded that whereas it is customary to think of air as pressing straight down, most individual air particles, in fact, strike the surface of the spaghetti obliquely. Those particles striking the spaghetti close to the point where it entered the mouth, and whose vector had some inward-pushing component, would force it in. Exactly how close to the mouth the particles would have to hit would of course depend on whether the spaghetti was al dente or boiled to within an inch of its life. You want to take it up with the sci.physics crowd, be my guest.

Sucking up

Dear Cecil:

Mama mia, Cecil! The sci.physics crowd's pathetic shot at explaining how anyone can suck up spaghetti makes one wonder why our tax dollars are still subsidizing hopeless efforts at science education. We might as well be funding pictures of naked thermocouples.

These wonks have missed the point entirely. Spaghetti sucking is not merely a function of the difference in pressure between the outside of the mouth and the inside. You can easily recognize this using a simple thought experiment. Imagine that a physicist--Erwin Schroedinger will do as an example--places the end of a strand of cooked spaghetti in his mouth. Then, instead of sucking, imagine that he turns on a pump that rapidly increases the air pressure surrounding his head.  The air-pressure differential will, according to probability, crush Schroedinger's skull like an eggshell long before it neatly forces the limp spaghetti through his pursed lips. (I encourage any sci.physics wonks who doubt this to try the experiment at home.) Ergo, it's not just air pressure.

A closer approximation to the right answer is that the spaghetti strand has its own density and cohesion. When you suck, the difference in density between the spaghetti strand and the air inside your mouth pulls the molecules on the surface of the spaghetti into your mouth; because the spaghetti's cohesion holds it together, the rest of the spaghetti is pulled in too. But then, I'm sure you knew that all along.

Dear Harry:

When will I learn? I thought this question, while not without interest, was too esoteric to get much reaction, forgetting that giving the Teeming Millions a question that's esoteric is like throwing the piranhas raw meat. I got rants about spaghetti sucking from every unemployed Ph.D. in North America, plus a few who aren't unemployed but who--how shall I put this?--probably ought to keep their resumes up-to-date. A sampling of alternative theories, starting with yours:

"When you suck, there is a difference in density between the spaghetti strand and the air inside your mouth."

I never suck, although I have my off days. What does "a difference in density" have to do with it? There is a difference in density if the stuff just lies there on the freaking table. We're talking about spaghetti, not an ideal gas.

"Spaghetti isn't a solid (although one can use air pressure differential to pull a round solid into the mouth, but at nowhere near the spectacular rate at which spaghetti can be sucked, as it were) but a starchy gel and so subject to physical laws that govern both solids and liquids. When subjected to a pressure differential, the spaghetti strand necks down slightly and . . ." (Brian).

OK, the spaghetti isn't solid but mushy. So what? You can suck solid things too. Cecil has been . . . well, I was about to say Cecil has been sucking various cylindrical objects, but I recognize that a certain element will find this comical. Let us pause while the lads get it out of their systems. Very well. By process of experiment, we learn that the speed at which something can be sucked depends very little on whether it's solid or spongy--mostly it's a matter of its weight relative to its cross section. I therefore feel entitled to ignore the fluidic aspects of the situation.

"The air pressure around the spagetti causes an internal pressure in the spagetti. At the interface between outside and inside your lips, the internal pressure of the spagetti drops; the spagetti flows down this pressure gradient" (Bruce Kline).

Bruce. It's "spaghetti." Quite a few people persuaded themselves that spaghetti exhibits characteristics of flow and that it somehow extrudes into your mouth. But this could not result in motion of the spaghetti without loss of structural integrity or at least permanent deformation, which does not occur, and in any case would be slow. This is spaghetti, not lava.

"I'm going with the dragging-it-along theory, wherein the seal of the lips around the strand is not perfect but allows some air or, more probably, a thin layer of spaghetti sauce to be sucked in, dragging the strand along with it" (J. Ebert).

First thing I thought of. First thing I rejected. The sauce acts as a lubricant, sure. However, from our experiments with nonpasta above, we conclude that an extremely thin film of spit is sufficient to facilitate sucking. Given the minute amount of fluid and the minimal cross section it presents, it's implausible to suggest that the sauce or whatever is the sole or even the primary medium of propulsion.

Cecil must be really desperate if he's consulting the noodleheads on Usenet.

Not at all. Cecil loves the noodleheads on Usenet. It's like consulting the regulars at the local bar. The quality of advice is easily as good, and you don't have to stand for the price of drinks.

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