Did Bilbo accidently slip on the ring, or did he purposely do it right when he saw the elves and that they just didn't notice?
On the first page of Chapter 9 it says, "So they simply stopped dead and sat down and waited - all except Bilbo, who popped on his ring and slipped quickly to one side." (p 185 of my book)
Mrs. S
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"The early bird gets the worm, but the second mouse gets the cheese."