From Alice's Adventures in Wonderland, and Through the Looking Glass, by Lewis Carroll
"In winter, when the fields are white,
I sing this song for your delight---
only I don't sing it," he added, as an explanation.
"I see you don't," said Alice.
"If you can _see_ whether I'm singing or not, you've sharper eyes than
most," Humpty Dumpty remarked severly.
-- Through the Looking-Glass
Chapter vi