You're not going out with him/her, but you do like them, and they like you too. Basically you act like you're in a relationship and get jealous when they talk to the opposite gender.
(In Kelly's head): Danny is so cute!! Aww we would look so cute together!!
Danny: Hey Kelly.
Kelly: OMGOMG ARHAERIHgshroihERK HI DANNY!
Danny: You cute. I'ma talk to Cindy.
Danny: Hey cindy, you hot.
Kelly: WTFFFF WE'RE IN A CRUSHATIONSHIP!! YOU BROKE MY HEART!
Danny: *just ignores* So yeah Cindy, Prom 201_?
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.