It's observation, inconspicuous as anything. It's knowing someone better than you know yourself, better than you or they will ever realise. It's being able to lightly tease them until the cows come home and them not minding because they know at the end of the day, you will be there for them when they need you. It's being honest for them because some things they cannot see for themselves.
It's sharing ice-cream on a Friday night, it's laughing until your laugh turns inaudible, it's looking over at them and nodding or raising an eyebrow and just knowing you're on the same wavelength. It's doing crazy stuff together, stuff you wouldn't be seen dead doing with anyone else.
It's when you can look any which direction and can immediately think of a memory. It's realising you would go to the ends of the earth if they asked you to. It's when all you want to do is call them and talk about everything and yet nothing at all.
It's what fills comfortable silences, it's as familiar as home, it's water fights in unforgettable summers. It's something you are incredibly lucky to experience and it's so fragile and so precious; it's like a single poppy in no-man's land.