The essence of grammar is recursion

Simple grammar

G = {N,V,S,P}

S ? aSa

S ? bSb

S ? c

E.g., c, aca, bcb, aacaa, aabacabaa

S => aSa => aaSaa => aabSbaa => aabaSabaa => aabacabaa

The man lit his awful sigar

The man that you thought was old lit his awful sigar

The man that you thought that your mother had seen lit his awful sigar

et cetera

It allows an infinite number of sentences to be generated by just a few rules