What is buckling? What should be the shape of a rod for preventing buckling?

(N/A) Buckling is a phenomenon where a structural member,such as a rod or column,undergoes a sudden change in shape (usually bending or bowing) when subjected to a high compressive load,even if the material has not reached its yield strength.
To prevent buckling,the rod should be designed to have a high moment of inertia. This is achieved by increasing the cross-sectional area away from the neutral axis. Therefore,a hollow rod or a rod with an $I$-shaped cross-section is more effective at preventing buckling than a solid circular rod of the same mass.