  Glasses are ubiquitous amorphous solids that remain one of the big mysteries
in condensed matter. Despite the vast body of literature on glasses, a unifying
approach to link the structure and dynamics of glasses is still missing. A
growing set of evidence, however, indicates the microscopic local geometry as a
key ingredient. This originated from the seminal work of Frank, who conjectured
that glasses may be the result of the local tendency of liquids to form
icosahedral structures, which are not capable of globally filling space
regularly. Here, we show that, for a fundamental glass model, dynamics can be
fully understood by simply counting the number of tetrahedra. Both globally and
locally, these local structures directly predict dynamical slowdown. After more
than 60 years of Frank's Conjecture, it might not be the icosahedra that matter
for glasses, but rather the tetrahedra inside them.
