Security

Our house has a big metal door. This door is never shut if we're home, except for when we're asleep. It also only has one doorknob. It's not on there with any bolts or other securing measure - you can slide it out one side and put it on the other really easily. 

So my family's version of "locking up" when we're not home in the daytime is taking the doorknob out of the inside, using it to close the door from the outside, and then "hiding" (I use this term loosely, since it's just sitting on some scarves or whatnot) it in the glass door cabinet we have outside next to the entrance of the house. 

Awesome.