Help! I've Been Locked Out! ...Of Everywhere
As anyone who lives in a German apartment for any amount of time will tell you, eventually you'll step outside, the door will close behind you, and you'll realize that German front doors have the wonderful ability to lock themselves without turning a key or latch on the inside. And also question why their kitchen has an old-timey key in the door, for what appears to be no reason. What's going on here?
German front doors are generally double keyed, with a keyhole on the inside of the door as well as the outside of the door. The door typically can't be opened from the outside when closed without a key, as the lock cannot be set to an unlocked state like a typical American door knob or handle. Depending on the particular lock you have, you may be able to simply leave a spare key on the inside of the door, and continue to unlock the door from the outside. However, before doing so, you should double check that you are able to, as otherwise, you may find yourself locked out, with a key, because the key on the inside keyhole is blocking you from unlocking your door.
In general, this system helps to secure your apartment or house, by guaranteeing that you can't manage to forget to lock your door when you leave. That being said, make it a habit to double check that you have a key on you on your way out the door, lest you end up having to make a call to a locksmith or roommate to try and get you back inside. Assuming you remembered your key on your way out, you can usually also turn the key an additional time around in order to extend the deadbolt in the lock.
Within your apartment, you'll likely have noticed that instead of having door locks or latches on the internal doors, many will have old fashioned keys instead (well, other than most bathrooms anyway). For doors to kitchens, living rooms, or bedrooms, you may choose to take the key out and leave it somewhere it won't get lost if you never intend to lock it. If you are living with roommates however, and you would like to be a little more security conscious, you can replace the lock inside your door without having to change the door handle in most cases.
To do so, you'll have to go to a lock shop, home improvement store, or look online for the internal unit of the lock (one with the more typical toothed key). With the door open, looking at the side of the door (the part that is flush with the door frame when closed), you'll typically see that the plate holding in the lock mechanism is held on with two screws. Remove the screws, slide out the lock, and replace it with the new unit you purchased. Keep the original lock however, as you'll be expected to leave the apartment in the condition you found it (key and all!) when you move out. Similar after-market knobs that can be used to replace the lock can also be found if you are so inclined.