Spinors and Octonions
  Octonions are introduced through some spin representations. The groups $G_2$, $F_4$ and the $E$ series appear in a natural manner; one way to understand octonions is as the "second coming" of the reals, but with the spinors instead of vectors. Some physical applications in $M$- and $F$-theory as putative "theories of everything" are suggested. 
Luis J. Boya (Universidad de Zaragoza)