It's Latin Grammar:
...Alice thought this must be the right way of speaking to a mouse: she had never done such a thing before, but she remembered having seen in her brother's Latin Grammar, `A mouse -- of a mouse -- to a mouse -- a mouse -- O mouse!' The Mouse looked at her rather inquisitively, and seemed to her to wink with one of its little eyes, but it said nothing...Thus...
A Pus
Of a Pus
To a Pus
A Pus
O Pus!