A lock keeps back us from entering a door. Key helps us to open the door. Then the important thing is the key must match the key hole on the lock. I feel the key hole is like a light bulb. Once you have an idea, the light bulb turns on, your mind opens the door for a new concept, a new world.
I think puzzle piece is the perfect example for negative space. It has different ins and outs which in a way like human. No one is perfect. Each has its own advantage and disadvantage. But when each individual, humans, animals, and plants, adds together, we make the whole and become the perfect.