If you know the officer insignia for the US Army you will understand this joke

A young Second Lieutenant approaches the crusty old CSM and asked about the origin of the commissioned officer insignias.
The CSM replied, "It's history and tradition ... First we give you a gold bar representing that you are very valuable and also malleable. The silver bar also represents significant value, but is less malleable. When you make Captain, your value doubles, hence the two silver bars. As a Colonel you soar over military masses, hence the eagle. As a General, you are, obviously, a star. Does that answer your question?"
"Yes," said the Second Lieutenant, "but what about Majors and Lieutenant Colonels?"
"That goes waaaay back in history ... to the Garden of Eden even. You see we 've always covered our pricks with leaves."