This comes up pretty often in (well-designed) video games. Say you have colored doors that can only be opened by keys of the same color. If you put the blue key

This comes up pretty often in (well-designed) video games. Say you have colored doors that can only be opened by keys of the same color. If you put the blue key on the path to the blue door, then players will never learn that you need the color of the key to match the color of the door. But if you put the blue key on a side path and have the player encounter a blue door relatively early on, then they will encounter the door first and cannot progress until they find the key. The frustration at not being able to progress is much more effective at teaching the player about the "colored keys for colored doors" mechanic than any amount of explaining you could have done.