Yes, you locked / Key

"Did I lock the door?"
"Yes, you locked."

Many people have same experience of locking the door. Sometimes they cannot figure out whether they lock the door or not, even though they just came out from their home. Then, they go back to the home and check. This key design is motivated from this situation. They don't have to take care of the problem any more.
This key helps you remember whether you lock the door or not. In the center of the key, there is a special indication that you locked the door. And the indication tell you did. This key concept will make you not to go back the home.