Riddle of the Day

[Gizmokid2005]

I'm white, round, but not always around.
Sometimes I am half, sometimes I am whole, sometimes a slice of me is all you will know. Sometimes I am light, sometimes I am dark, sometimes I am both. Everyone wants to walk on me but only a few ever have...
What am I?

ANSWER (Congrats TL!!): Moon.

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NEXT RIDDLE:

I can run but cannot walk,
sometimes I sing but I never talk.
I lack arms, but has hands;
I lack a head but has a face.
What am I?