I'm sure everyone's familiar with these, but what the hell:

There was once a man who said "God
Must think it exceedingly odd
If he finds that this tree
Still continues to be
When there's no one about in the Quad."

(Monsignor Ronald Knox)

This was swiftly replied to thus:

Dear Sir, Your astonishment's odd:
I'm always about in the Quad.
And that's why the tree
Will continue to be,
Since observed by Yours Faithfully, God.

(Anon.)