“Let us go then, you and I, When the evening is spread out against the sky Like a patient etherised upon a table”

The poem's opening proposes an evening outing and then compares the sky to a patient under anaesthetic, so the joke lands before the reader has any idea what kind of evening is being proposed. Eliot's deadpan medical image is the reason the poem is still genuinely funny, which few literary poems manage to be.