The Tail of the Sphinx

A DOG of a taciturn disposition said to his Tail:

"Whenever I am angry, you rise and bristle; when I am pleased, you wag; when I am alarmed, you tuck yourself in out of danger. You are too mercurial - you disclose all my emotions. My notion is that tails are given to conceal thought. It is my dearest ambition to be as impassive as the Sphinx."

"My friend, you must recognise the laws and limitations of your being," replied the Tail, with flexions appropriate to the sentiments uttered, "and try to be great some other way. The Sphinx has one hundred and fifty qualifications for impassiveness which you lack."

"What are they?" the Dog asked.

"One hundred and forty-nine tons of sand on her tail."

"And - ?"

"A stone tail."