The 고 clause is what happens just before opening. Four of these lines use a sound word plus the verb for doing, which is the standard way to attach a noise to an action: yanking a door open with a bang, popping a stopper out, opening it with a ta-da to find a fossil inside. Others are plain sequence, such as keeping it warm for a few days and then opening it, or stop talking and open up. One line is a false match on the noun for fever.