# Why do molecules vibrate?

Because the lowest temperature we've accomplished for molecules is not quite $\text{0 K}$. But seriously though, the question doesn't really have a true answer; this is like asking, "why do people breathe?".
Just as people breathe because breathing takes in the oxygen from the surroundings and cyclically incorporates it onto hemoglobin (so that eventually we breathe out ${\text{CO}}_{2}$ and breathe in oxygen again later)...