I turn around once. What is out will not get in. I turn around again. What is in will not get out. What am I?

Why this works

This riddle cleverly plays with the functions of a key in a lock. When you turn a key once, it either locks or unlocks the door: "What is out will not get in" means that if the door is locked, outside elements can't enter. Conversely, when you turn the key again, it locks the door, ensuring "What is in will not get out," meaning that once the door is secured, those inside cannot leave. The riddle captures the dual role of a key in controlling access!