RMT Basics, page 2

What did you do?!

In the process of breaking the pencil, you established this force pattern. The force "F" which you applied was resolved in the upper portion of the material of the pencil as compression -- the squeezing force, and in the lower portion of the pencil as tension -- the stretching force.

With the application of additional force, the material (wood) of the pencil succumbed to the stretching force (not the squeezing force). WHY? Because like most materials, wood is weaker in tension than it is in compression.