Re: Breaking uncooked spaghetti by bending always results in 3 pieces. Why?

Spaghetti is brittle but also elastic.

It is possible to break spaghetti into only two pieces by shearing rather 
than bending it.  Shearing in the sense that the two points at which the 
spaghetti is held are as close together as possible and the force applied 
is normal (at a right angle or 90 degrees) to the length of the spaghetti.

When you bend a piece of spaghetti, and do so slowly, you will notice the 
spaghetti bends at first, rather than breaking immediately.  With a high-
speed camera you should also be able to observe that the curvature is 
smooth right up to the instant of fracture.

Fracture occurs at the two points where the bending stress on the 
spaghetti is greatest.  There is pressure but only a very small bending 
stress in the two areas pressed between your fingers.  In the center piece 
of spaghetti, the third piece after the spaghetti breaks, the spaghetti is 
free to bend and spreads the bending stress.  Somewhere between the center 
and each held end, is a point where the spaghetti is least capable of 
handling additional stress.

Thanks for the question.  I had never considered spaghetti in quite that 
manner.  I actually raced to the cubbard and snapped a few pieces.  If you 
try shorter pieces, you will find the above does not work as well.  In 
some instances you get two pieces but the break is to one side and in 
others you get four pieces.  That is because the shorter the piece, the 
less significant the curving of the spaghetti, as described above.

As a reference I used A Textbook of Materials Technology by Lawrence H. 
Van Vlack, but it doesn't say a lot that is directly relevant to what I 
have described above.  I also did not have a lot of luck finding a 
spaghetti brittle-fracture reference on the web, but did find a 
description of a balloon poping experiment.  Though the poping of a 
balloon is much more complicated, spaghetti breaking and balloon poping 
have a lot in common.
 http://www.balloonhq.com/faq/howpop.html

sid
The following website describes a balloon poping experiment.