How does your submission match the theme?
I designed a system where the cycle of water is the main mechanic. As the dual purpose design I will explain how my game works:

The player is a cloud who has to be alive as long as he can in a forest with lots of fire. The only actions are movement and rain.


RAIN (LOOSE WATER)
When the player(cloud) rains several things happens:

- If it rains long enough in a fire thats kill the fire
- As the cloud loose water have less weight so it can move faster
- Rain clouds usually are lower in height, so when it rains it also move upwards
- If it rains on a baby tree(root) for a while a new tree will grow in the place
- But If you rain all your water (you have a water bar) you loose the game


GET WATER
There are two ways of regain water for your cloud:

- There are several big clouds that act like tanks, are quite high so when you are short on water (you are high) you can go fast to a tank an refill yourself quite a lot.

- Each time you rain on a fire and extinguish it you gain some water because the fire evaporates some of your rain.

- While there are trees burning you gain a little bit of water overtime


BURN
- If a fire touch a tree it burns it, and creates a new fire, but cant burn the roots.


- Each action have several purposes for the cloud, so it has to choose strategically and with patience when and where to rain, because that act have many consecuences. Usually the player loose because it loose his water trying to do everything as fires grow in number.

- Even the fires burning a tree have good and bad consecuences for them and also the tree becomes a new danger for their fellow trees.

- In general I create a full system where a simple action like rain have lots and complex purposes that the player have to balance.

- I also had another element in mind, a cauldron, that would refill the cloud tanks only when the fires burn their water, but also the cloud-player would have to rain on the cauldron to refill their water. Sadly I didnt have time enough to develop that, maybe in the future :)

Third-party resources
I made all the assets (3D models, scripts, textures, particles, etc) except:

- Projector shader and blob shadow from Standard Assets
- Free Post Processing effects package from Unity
- FontdinerSwanky font from Google Fonts

Contributors
I only ask for my friend Paula how to use some channels from Adobe Photoshop in the creation of one particle texture. Thats it.