Ruby is known as a good language for DSL (Domain Specific Language). However, it is unclear what differentiate internal DSL and normal library. This presentation tries to explain the difference. Both DSL and normal library solves part of programmers task. They tries to make programming easier. However, DSL tends to use some kind of black magic unlike normal library. The black magic is used to realize succinct and easy-to-read program for many applications in the domain. I hope this presentation makes programmers aware to design good library.