Cerberus

Dear Reader, should you chance to go
To Hades, do not fail to throw
A "Sop to Cerberus" at the gate,
His anger to propitiate.
Don't say "Good dog!" and hope thereby
His three fierce Heads to pacify.
What though he try to be polite
And wag his Tail with all his might,
How shall one amiable Tail
Against three angry Heads prevail?
The Heads must win.--What puzzles me
Is why in Hades there should be
A Watch dog; 'tis, I should surmise,
The last place one would burglarize.