The Sphynx leers at you, and says, "If you wish to enter the Tunnel of Riddles, you must first prove your worth. If you do so, I will allow you to enter the tunnel. Fail, and you will become my dinner."
He clears his throat, and in a clear, melodic voice, says:
In the caves I am a ghostly cry
In the sky I accompany birds on high
The willows weep before my might
With Fire I set whole forests alight
What am I?
The Sphynx stretches, and opens his gaping maw in what could be taken for a yawn. He then sits back and silently watches you.