This requires a dry hen's egg at room temperature. Hold an egg near a candle flame to cover it with soot. It will need to be completely covered. This is tricky, because if the egg is a tiny bit damp the soot will easily flake off on to your fingers as you turn the egg. Once the egg has a nice black sooty coating, gently immerse it in a bowl of water.
If it is completely covered, the carbon in the soot repels the water and holds a fine film of air. This will give the egg a silvered mirror appearance.

http://www.youtube.com/watch?v=wRVaeL8pWiI

That's a great illusion along with many others on that YouTube channel.