Click on the materials on the right to use them to paint your own world. Lava can flow over things and grass turns into dirt when placed under another block. But you can turn both of those settings off by tapping on the gear icon in the top right. This is still in the early stages, a lot of the code is pretty janky, I'm planning to add aromatic world generation, make it playable, be able to load up past worlds and so much more!
If you have seen this game under "StarScratcher999" that is because it is my other account.