"gay as a lark"
Simile is the comparison of two unlike things using like or as. Related to metaphor
Example:
He eats like a pig. Vines like golden prisons.
Poetry is, first of all, a communication - a thought or message conveyed by the writer to the reader. It is not only an act of creation, but an act of sharing. It is therefore important to the reader that he understands how the poet uses words, how he puts fresh vigor and new meaning into words. The reader's understanding is immeasurably increased if he is familiar with the many techniques or devices of poetry. Some of these are extremely simple; a few are rather elaborate.
The simplest and also the most effective poetic device is the use of comparison. It might almost be said that poetry is founded on two main means of comparing things: simile and metaphor. We heighten our ordinary speech by the continual use of such comparisons as "fresh as a daisy," "tough as leather," "comfortable as an old shoe," "it fits like the Paper on the wall,"
"gay as a lark," "happy as the day is long, pretty as a picture." These are all recognizable similes; they use the words "as" or "like."
http://www.tnellen.com/cybereng/lit_terms/simile.html