Add clarification

Author: afontcu

src/guide/write-easy-test.md

@@ -30,7 +30,7 @@ Think in terms of inputs and outputs. Roughly, this is everything you should tak
 
 Notice how this list does not include elements such as internal methods, or intermediate states or data.
 
-The rule of thumb is that **a test should not break on a refactor**, that is, when we change its internal implementation without changing its public API. If that happens, it might mean the test relies on implementation details.
+The rule of thumb is that **a test should not break on a refactor**, that is, when we change its internal implementation without changing its behavior (that is, without changing its public API). If that happens, it might mean the test relies on implementation details.
 
 
 ## Build smaller components