diff --git a/challenges/advanced-typeguard/question.py b/challenges/advanced-typeguard/question.py
index 7f0924c..dde05bf 100644
--- a/challenges/advanced-typeguard/question.py
+++ b/challenges/advanced-typeguard/question.py
@@ -1,16 +1,14 @@
 """
 TODO:
 
-is_string is a function that takes an argument value of arbitrary type, and returns a boolean.
-You should make is_string be able to narrow the type of the argument based on its return value:
-when it's true, narrow value's type to str.
-Basically, it should work like `isinstance(value, str)` from the perspective of a type checker.
+`is_string` determines whether the input value is a string.
+Your job is to make the type checker be aware of this information.
 """
 from typing import Any
 
 
 def is_string(value: Any):
-    ...
+    return isinstance(value, str)
 
 
 ## End of your code ##
diff --git a/challenges/advanced-typeguard/solution.py b/challenges/advanced-typeguard/solution.py
index 301ff68..e848bce 100644
--- a/challenges/advanced-typeguard/solution.py
+++ b/challenges/advanced-typeguard/solution.py
@@ -1,10 +1,8 @@
 """
 TODO:
 
-is_string is a function that takes an argument value of arbitrary type, and returns a boolean.
-You should make is_string be able to narrow the type of the argument based on its return value:
-when it's true, narrow value's type to str.
-Basically, it should work like `isinstance(value, str)` from the perspective of a type checker.
+`is_string` determines whether the input value is a string.
+Your job is to make the type checker be aware of this information.
 """
 from typing import Any, TypeGuard
 
diff --git a/challenges/extreme-self-casting/solution.py b/challenges/extreme-self-casting/solution.py
index be906ad..93822ba 100644
--- a/challenges/extreme-self-casting/solution.py
+++ b/challenges/extreme-self-casting/solution.py
@@ -13,8 +13,8 @@
 from typing import Callable, Concatenate, Generic, ParamSpec, TypeVar
 
 
-R = TypeVar('R')
-P = ParamSpec('P')
+R = TypeVar("R")
+P = ParamSpec("P")
 
 
 class Fn(Generic[R, P]):