Ruby inside of Ruby. Ruby can run itself, allowing extensibility.

Ruby Programming Language

Hello World


Ruby on Rails


You might not be able to guess where you'll use this, but it surprisingly comes in handy often when programming. Metaprogramming is the art of programming inside your programming language. For example, executing a script your user types in on-the-fly (hopefully you trust them), which might be good for a web-based administration panel, for example.

puts "Hello World"
eval("puts 'Hello World'")

This allows you to do all sorts of fun things, but could potentially be dangerous as it can execute arbitrary code on your machine.