|A few pieces of spaghetti|
Take a piece of spaghetti, hold onto each end, slowly bend it until it breaks.
How many pieces does it break into?
You should find that the spaghetti breaks into 3,4 or even 5 pieces but hardly ever 2
When the spaghetti breaks the movement caused by this break leads to another part of the spaghetti becoming more curved. As the whole piece was already at its limits and just about to break this extra curvature causes the spaghetti to snap. Sometimes this second break can trigger a third or even fourth break.
Exactly what is going on has challenged some of the best minds in physics for decades (including nobel prize winning, and bongo playing Richard Feynman). But recently some french physicists using high speed cameras and a lot of maths and computers have worked out exactly what is happening (http://www.lmm.jussieu.fr/spaghetti/publi.html)
To a rough approximation this is what is going on:
If you bend a ruler on a table and then release it cleanly it will jump up into the air as it straightens. This is because in order to straighten it has to get longer, the table is in the way, so it pushes on the table. Newton's second law says that if you push something it pushes back, so the table pushes it up into the air.
|If you bend a ruler it pushes down and the table pushes back.||As the ruler is released it straightens, keeps pushing down and gets pushed upwards by the table||It keeps on moving up and flies into the air|
When spaghetti breaks it straightens, and this straightening pushes the still bent spaghetti back. Eventually this increases the curvature enough to snap the spaghetti in a second place, breaking it into 3. This second break can then trigger a third or fourth, depending on when and where it broke the first time, and any weaknesses in the spaghetti.
|The spaghetti starts off bent||It then fails somewhere||As it straightens it gets longer pushing the still bent spaghetti back.|
|This will increase the curvature until something breaks||The broken bit then flies off.|
In reality it is more complex, and depending on where and when it breaks in the first place the subsequent breaks can change.