Sand will spread to adjacent cells, so you need to have an "enclosed" area to have cells without sand.
Thank you!
Is discovery of the mechanics supposed to be part of the puzzle? It would still be a challenge even if you gave the player an initial green square. Anyway, it’s a fun puzzle, so thank you again!
To 5anz and Tstaffor and xxuurruuii: Thank you for trying to help me. Apparently, I need to be spoonfed sometimes :)