Numerical simulations aim to represent the real-world dynamics of a physical system with the help of a computer. Despite being an incredibly complex machine, a computer is, at its core, an ultra-fast calculator with increasing but finite capacity. In contrast, real-world systems are infinite: whatever level of zooming you look at, one always finds that space is continuous. The mathematical modeling alleviates this issue in its final steps by considering only slices of space (and time).