It is basically fracturing, but I do have some more about interactions of molecules (what fractures and, actually, what happens similarly in phase transitions--such as ice melting).
Molecules interact with each other through several different ways:
1) Hydrogen bonding.
This occurs mainly due to electronegativity of molecularly bonded atoms. Atoms to the right of the periodic table (excluding noble gases) have negative charges as ions, and atoms to the left of the periodic table have positive charges as ions (gaining and losing electrons in solution, respectively). Two very differently charged atoms form ionic compounds. Those with a lesser difference of electronegativity form molecular compounds, such as water. However, these differences in electronegativity result in partial negative and partial positive charges in atoms in the molecule, as the shared electrons spend more time around atoms with more electronegativity (such as oxygen) than around atoms with less electronegativity (such as hydrogen). Because of this partial charge, an electronegative atom in a molecule (especially nitrogen and oxygen) will interact weakly with other molecules with hydrogen (which has a partial positive charge in most compounds).
Depending on the number of these connections (and the ones below) in a compound such as ice and the angles at which these connections (and the ones below) are formed, solids acquire properties related to how much applied force is necessary to break a substance made of these molecules.
Also, these intermolecular interactions are what are broken when a substance is heated and when it undergoes a phase transition (vaporization, freezing, sublimation...)
2) Polar interactions.
These are similar to what is described above (electronegativity differences within a molecule as a result of unequal electron sharing), except that these do not involve hydrogen atoms. These are also weaker bonds than hydrogen bonding.
3) VanDerWaals interactions.
These occur when compounds have only similarly electronegative atoms in their molecules, such as mixtures of gasoline (hexane, octane... in which molecules are only made of hydrogen and carbon atoms). These are the weakest interactions and become important in organic chemistry (as does hydrogen bonding).
These are some important concepts in organic chemistry, physical chemistry, and mechanical engineering, if Wolf wants to learn more than this. Most textbooks will likely have much too complicated math as an explanation but tend to have lots of good pictures and graphs demonstrating these principles. (FYI, this conceptual level is about what is expected in introductory college chemistry and physics courses).
I hope that helps!
