Greg Cobb, age 14, of Santa Maria, California, for his question:

How is old paper currency removed from circulation?

The average dollar bill gets passed from hand to hand, stuffed into wallets and cash registers, rolled into wads and sometimes crumpled into pocket corners. True, it is made of superior paper, calculated to withstand a lot of wear and tear. Nevertheless, its average life expectancy is no longer than about 18 months. During its life time of busy commerce, it makes frequent trips to and from local banks. For its first few months, tellers pay no attention as it is paid in and paid out. But as it shows signs of wear and tear, somebody in the bank begins to cast a suspicious eye upon it.

Maybe the next or the next time, the shaggy old bill fails to pass its bank inspection. It is taken out of circulation and stacked with other worn out paper currency. The local bank ships it to a Federal Reserve Bank. There the silver certificates and United States notes are burned. New replacements are sent back to the local bank. Federal Reserve notes are shipped to the Treasury Department and burned to ashes    naturally under maximum security.