I like working on a physics engine. Simulated things feel so real, as if you can touch it, play with it.

Using Euphoria To make realistic Roblox physics animations

How to Create a Custom 2D Physics Engine: Friction, Scene and Jump Table | Envato Tuts

Coding a 2D physics engine from scratch — and using it to simulate a pendulum clock

Creating a Game Physics Engine with C

How to Create a Custom 2D Physics Engine: The Basics and Impulse Resolution | Envato Tuts