I shit you not. Just spoke with the mailman about it today.
In the box is the key to the building. That's why the mailman doesn't have to carry keys for every building on the route.
I grew up in a house, so I just never thought about how this works until my girlfriend pointed it out.
I realized this before going to sleep a couple of days ago.
Example 9x2=18; 1+8=9. 9x123=1107; 1+1+0+7=9.
Also: 9x11=99; 9+9=18. BUT 1+8=9!