Answer:
In the night a mountain, in the morning a meadow.
I am never quite what I appear to be. Straight-forward I seem, but it's only skin deep. For mystery most often lies beneath my simple speech. Sharpen your wits, open your eyes, look beyond my exteriors, read me backwards, forwards, upside down. Think and answer the question...
What has 13 hearts but none beat?
What can burn the eyes, sting the mouth, yet be consumed?
Something wholly unreal, yet seems real to I. Think my friend, tell me where does it lie?
You use it between your head and your toes, the more it works the thinner it grows.