  The glass transition is a long-standing problem in physics. Identifying the
structural origin of the transition may lead to the ultimate solution to the
problem. Here, for the first time, we discover such a structural origin by
proposing a novel method to analyze structure-dynamics relation in glasses. An
interesting two-step glass transition, with rotational glass transition
preceding translational one, is identified experimentally in 2D colloidal rod
systems. During the transition, parallel and perpendicularly packed rods are
found to form local free energy minima in configurational space, separated by
an activation barrier. This barrier increases significantly when rotational
glass transition is approached; thereby the rotational motion is frozen while
the translational one remains diffusive. We argue that the activation barrier
for rotation is the origin of the two-step glass transition. Such an activation
barrier between well-defined local configurations holds the key to understand
the two-step glass transition in general.
