Just take a look at how specs [1] works:

  "Some element" should {
      "behave in some way" in {
          // ...
      }
  }

According to this, the String type has a "should" method and an "in" method, but it really doesn't. Instead, there is an implicit conversion from a string to another type that actually has those methods. They idea here is to make the DSLs more fluent by removing the need to worry about complex type interactions while designing the interface. It lets the users use the pre-existing types they're already familiar with.

[1] http://code.google.com/p/specs/