A cardkey. They open doors many times, but almost never will a cardkey actually have the ability to lock a door.
A road map. You might tell someone to open up a road map, and they'll unfold it. But you'd never say "close the map." You'd tell them to "refold" the map. So we use the term "open", but not "close."
A capital letter. It opens lots of sentences, but they always close on a period, question mark, or exclamation point.
"To Whom It May Concern" opens a lot of letters, but never closes them.
A coconut on the tree. First, the harvesters open the outer green husk, which we never see in the store. Then, you open the brown fuzzy coconut.
