This document discusses building the Ruby programming language in Smalltalk. It addresses some of the key challenges in doing so, including differences in the compilers, handling different argument arities between Ruby and Smalltalk methods, allowing Ruby and Smalltalk code to coexist, handling per-instance behavior, and tracking per-instance variables in Ruby. Synthetic "bridge" methods are discussed as one approach to addressing differences in argument arities. The document also shows how the number of core Ruby tests passed has increased over time, suggesting the choice to implement Ruby in Smalltalk was a good one.