When Mary gets home from work and goes to her PC to check e-mail, the PC isn't just sitting there. It's working for a biotech company, matching gene sequences to a library of protein molecules. Its DSL connection is busy downloading a block of radio telescope data to be analyzed later. Its disk contains, in addition to Mary's own files, encrypted fragments of thousands of other files. Occasionally one of these fragments is read and transmitted; it's part of a movie that someone is watching in Helsinki. Then Mary moves the mouse, and this activity abruptly stops. Now the PC and its network connection are all hers.

This sharing of resources doesn't stop at her desktop computer. The laptop computer in her satchel is turned off, but its disk is filled with bits and pieces of other people's files, as part of a distributed backup system. Mary's critical files are backed up in the same way, saved on dozens of disks around the world.

   
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