While their size and density is also an area for concern, it is more along the lines of the lack of physical attachment to anything.

How in the world did I ever begin talking about Brettski's marbles?

JHAP
~~~~~~~~~~
"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)