1. You literally know all of each other's secrets.
2. And though you may jokingly threaten to reveal them, you both know you never would.
3. On more than one occasion, you've hung out with your BFF's parents on your own.
4. You can hold a conversation without uttering a single word.
5. Something will happen, and at the same time, you'll both say to each other, "Remember that time...?", and then dissolve into laughter.
6. In fact, you'll often both say the same thing at the same time.
7. Because you're both always on the same wavelength.
8. You know that there are some things that you can tease them about...
9. You know all of each other's flaws and weird habits that no one else sees.
10. You can go two months without speaking and it's like no time has passed.
11. And you both know that even if you lived in different states, you'd still be just as close.
12. You can tell them literally anything without fear of judgement.
13. You have strange nicknames for each other.
14. You insult each other on a regular basis.
15. You've probably had at least one physical fight.
16. And you've had a ridiculous number of fights over completely stupid things.
17. You've worn couple's costumes for at least one party.
18. You've also accidentally both worn the exact same thing without meaning to.
19. You've definitely stolen each other's clothes, even if you're the opposite gender.
20. You either text each other non-stop all day, or you don't text for two whole weeks.
21. You have some cracker photos together that you've both vowed will never see the light of day.
22. You've seen each other have a drunk vom on many occasions.
23. You have the same favourite shows.
Basically because you force your BFF to watch all the shows you love, and vice versa.
24. If you don't already know them, you could definitely guess your BFF's passwords.
25. You always, always know when they're lying.
26. They are the ONLY person who can talk shit about your family.
27. Your inside jokes have progressed to the point where they no longer make any sense.
28. When they get a partner, you'll tease them mercilessly for all the mushy couple things that they do.
29. But when your BFF gets a new friend, you get weirdly jealous and protective.
30. Though you know that there is actually nothing that could ever come between you.
31. Because 20 years is a really, really long time.
