Here's why:

* Chirality: Most amino acids are chiral, meaning they have a non-superimposable mirror image (like your left and right hands). This is due to the presence of a carbon atom (the alpha carbon) bonded to four different groups: an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a side chain (R group).

* Glycine's Structure: Glycine is unique because its R group is simply a hydrogen atom. This makes its alpha carbon bonded to two hydrogen atoms. As a result, it lacks the four distinct groups needed for chirality.

* Optical Inactivity: Without a chiral center, glycine does not rotate plane-polarized light and is therefore optically inactive.

Let me know if you'd like to explore more about chirality and amino acids!