Lock Before Key

Here's a recent addition to My Game Design Principles:

Present the player with the lock before the key to open it. A key without context is just trash. "What's this key for? I don't need it."

A lock can be intriguing. "What’s behind that door? What’s inside that chest?" Locks ask questions that only keys can answer.

Locks can take many forms: impassable rivers, high cliffs, invulnerable monsters. Keys also: rafts, jumping boots, flaming swords. Provide the need before the solution.

Repeat after me, lock before key.