You are an expert game maker. I want to build a 2D platformer game in Python using Pygame. The main character is a cute cavapoo dog.

Before writing code, please think step-by-step. Explain how we will build the player movement and the gravity first.

Game Rules:
1. The dog moves left and right with arrow keys.
2. The dog jumps with the space bar.
3. Create a green floor for the dog to stand on.
4...