Does anyone know of a general & special relativity simulator? That is, a program where I can setup various "thought experiments", by placing clocks and masses a

Does anyone know of a general & special relativity simulator? That is, a program where I can setup various "thought experiments", by placing clocks and masses at various point in space, where they can have various acceleration profiles, and can communicate with each other, and run the scenarios? I never really understood the twins "paradox" and the explanations for the source of the asymmetry seemed hand-wavy and "because".