Here's something they're virtually guaranteed not to have: a genuine 13th century copy of the Magna Carta. The original Magna Carta (the name means "Great Charter" in Latin) was signed in 1215 by King John of England, under heavy pressure from barons who were tired of paying for his wars. Regarded as one of the keystones of liberty, it enunciated principles that later would find a place in the U.S. Constitution,. Some 400 copies of the charter were made and distributed throughout England, but fewer than 20 of those have survived. The only copy in private hands will be auctioned by Sotheby's on Dec. 10. The copy is owned by the Perot Foundation, which purchased it in 1984. Proceeds from the auction will go to charity. Make sure there's plenty of cash in your checking account: A Sotheby's spokesman said the document is expected to sell for between $20 million and $30 million.