this is useful for many things. i open doors at my school like this so i can steal stuff like money, laptops, and a whole lot of other stuff.
what you need.

-a plastic card that is sturdy but flexable
-a screwdriver
what to do.
most doors open and swing towards the inside. with these doors there is a shield to prevent you from just sticking a card in and opening the door. Take a screwdriver and pry the door a couple of millimeters away from the shield. jab your card in the dor and move it up and down till you feel the metal knob or whatever it is that keeps a door from just opening. take your card out and remember where this spot was.
jab your card into the door where the knobwas and work the card back and forth while applying slight pressure to the door. eventually the metal thingy will slide into the door and the door will open. if it is your first time it will take a while but after doing this a lot you will do this very quick.

i can do this in under 2 sec cuz i am familiar where to push the card.