Why are four-leaf clovers considered so lucky? Because they are so very rare - occurring in only about 1 out of 10,000 clovers specimen.

Related Fact: What's the most clover leaves anyone has found? In 2009, Shigeo Obara of Japan found (well, created) a 56-leaf-clover through natural crossbreeding. He'd previously been recognized by Guinness for an 18-leaf-clover found in 2002. Obara died in 2010.

[Sources: Science Base, Japan Today and Guinness World Records. See previous Numbers of the Day here.]