Father: Son, if you ever find yourself needing an eraser, ask a Conservative.

Son: Why a Conservative?

Father: Because Liberals don't carry erasers.

Son: But aren't Liberals wrong about almost everything? I'd think they'd all have erasers.

Father: No, son. Erasers aren't carried by those who make mistakes; they're carried by those who are willing to correct their mistakes.

(Adaptation. Original Source Unknown.)