鍵

1. Origins, structure, and basic meaning

• Structure: 鍵, read かぎ, means a physical key for a lock and, figuratively, a crucial factor that makes a solution possible.
• Basic usage: It can lock an entrance door or identify a discovery as the key to solving a problem.

2. Common pitfalls and similar expressions

• A physical key and a key to a solution
  • It can be the physical key to a door or a discovery that becomes the key to solving a problem. The latter is not an object used in a lock.