An acrostic by Robert Blackwell, 1868:

Turn this book and at us look,
Heed our features, too,
Expressive, fine, our faces shine,
To please such folks as you;
With heads but four, we want no more,
Our eyes give us no light;
Our ears are deaf, but yet no grief
Disturbs us day nor night;
Deprived of feet we can not walk
In houses where we go,
The reason why we do not sigh,
Is left for you to know.
Ever free from care are we,
So turn this book, and at us look.

Reading the first letter in each line produces the phrase “The Two Oddities.” Inverting the book gives the answer to the riddle: The “four heads” are actually one carefully devised figure — each face is the other upside down: