Hi! I m Jack. I m not familiar with round knob door locks.?

Recently I have been shifted to a hostel and my room has a round knob door lock. It has a keyhole on the exterior and a push button on the interior part of the lock. For locking from inside, I just push the button present at centre of the knob. But I dont know how to lock it from outside. I inserted the key to the keyhole and turned it 90 degree either to left or right but it doesn't work. Help.