Another shot at clarity

src/doc/book/lifetimes.md

@@ -56,7 +56,7 @@ To fix this, we have to make sure that step four never happens after step
 three. The ownership system in Rust does this through a concept called
 lifetimes, which describe the scope that a reference is valid for.
 
-**Note** It's important to understand that lifetime annotations are _descriptive_ not _prescriptive_. This means that the lifetimes of references are determined by the code, not by the annotations. The annotations, however, point out the lifetimes to the compiler in case it can't figure them out by itself.
+**Note** It's important to understand that lifetime annotations are _descriptive_ not _prescriptive_. This means that who long a reference is valid  is determined by the code, not by the annotations. The annotations, however, point out this fact to the compiler in case it can't figure it out by itself.
 
 When we have a function that takes an argument by reference, we can be
 implicit or explicit about the lifetime of the reference: