Here's one I made up a while ago:

I can live for millions of years, but I am constantly changing.
When you try to grasp me, I slip through your fingers.
If you did grasp me, I would be no more.
What am I?
afghooey Reviewed by afghooey on . old old riddle... no cheating! whoever gets it first will get rep. points. what can you put in a barrel, that you can't see, that will make it lighter. good luck. Rating: 5