When __ bumps ___
Every time one object collides with the other object (or edge of the screen) the blocks inside will run. Tap each bubble to choose which objects should trigger the rule. "Self" refers to that specific object, rather than its clones. If you use self, you can copy and the paste the same rule into a bunch of objects without changing the code.
For an example, search the community for Flappy Bird. When the bird bumps into a pipe or the edge of the screen, the game is over.