In one sentence, describe what the following article is about:

Saying goodbye is actually a unique opportunity to think back on your friendship and tell your friend how much it has meant to you. It's not often friends get the chance to do that, so take advantage. Writing it all down in a letter is cathartic, and your friend will be able to treasure it forever.  Write down your recollection of how you met. Do you remember the first time you and your friend hit it off? Sometimes you just know when someone you meet is going to end up being a great friend. Write down some of your favorite memories. Recall movies you saw together, concerts you attended, fun sleepovers you had. If you want, open your heart even more. You can talk about the bad times, too, and how much the two of you have grown as friends as a result of overcoming them. Saying goodbye is sad, but it's something you'll both get through. Your friend will have a lot of adjustments to make, and as her BFF it's your job to cheer her up. Throw in some lighthearted stories or jokes that will make her laugh when she unfolds your letter.  Draw funny cartoon scenes of the two of you together. Write something in code and include a decoder on the back of the letter. with a promise to stay in touch. Make sure your friend knows that to you, this isn't the end of your friendship. You'll always remember the great years you spent together living in the same town, and this is simply the next phase of your friendship. Now you get to visit each other's cities and share what you're learning as you both explore different corners of the world.

Summary:
Reflect on your friendship. Lighten the tone with some jokes.  End the letter