Abraham Lincoln had what they called his "portable office"—his stovepipe hat. Inside it, he kept a letter from his "little correspondent," Grace Bedell. She was eleven years old, from New York. In 1860, while Lincoln was running for president, her father went to a county fair and brought home a campaign broadside, like a poster.  It had an image of Lincoln and his running mate, Hannibal Hamlin. Grace asked if she could write a letter to Lincoln. 

She wrote a letter that made it to Lincoln and he answered it; and they became pen pals. In her letter, with the audacity of a child, she tells him “You are unattractive. Your face, it's too thin, it's too narrow, you should grow whiskers to fill up the face and hide it, you know, that ladies will like you if you grow whiskers”  She misspells everything.

Lincoln writes back the most charming letter: "My dear little Miss Bedell, having never worn whiskers, do you think it would not be a silly piece of affection if I started growing them now?" But he thought about it, and that is why he grew the beard. 

The Lincoln who campaigned had no beard; the Lincoln who showed up at the inauguration did. He kept Grace’s letter in his top hat, his portable office. He would often take it out and say, "Looky here," reading it to lighten the mood before putting it back. It is an adorable story about Lincoln and Grace becoming pen pals, and why he grew his beard.