diff --git a/Cargo.toml b/Cargo.toml
index 80a59ec..280b42e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,3 +14,4 @@ lazy_static = "1.4.0"
 pest = "2.5.1"
 pest_derive = "2.5.1"
 rayon = "1.6.0"
+ntest = "0.9.0"
diff --git a/src/assignments/assignment12.rs b/src/assignments/assignment12.rs
index ec381ee..73d4d84 100644
--- a/src/assignments/assignment12.rs
+++ b/src/assignments/assignment12.rs
@@ -17,7 +17,7 @@ pub fn pong(rx1: &mut Receiver<u32>, tx2: &mut Sender<u32>) -> bool {
     todo!()
 }
 
-/// Executes the given functions (f1, f2) and returns the results.
+/// Executes the given functions (f1, f2) in concurrent and returns the results.
 pub fn use_scoped_thread<'scope, 'env, T1, T2, F1, F2>(
     s: &'scope thread::Scope<'scope, 'env>,
     f1: F1,
diff --git a/src/assignments/assignment12_grade.rs b/src/assignments/assignment12_grade.rs
index 332fe58..decd922 100644
--- a/src/assignments/assignment12_grade.rs
+++ b/src/assignments/assignment12_grade.rs
@@ -1,5 +1,6 @@
 #[cfg(test)]
 mod test {
+    use ntest::timeout;
     use super::super::assignment12::*;
 
     use std::sync::mpsc::channel;
@@ -41,4 +42,43 @@ mod test {
             });
         }
     }
+
+    #[test]
+    #[timeout(5000)]
+    fn test_scoped_thread_concurrent() {
+        use std::sync::Mutex;
+
+        let m = Mutex::new(0);
+        let (r1, r2) = thread::scope(|s| {
+            use_scoped_thread(
+                s,
+                || {
+                    for i in 0..100 {
+                        loop {
+                            let mut a = m.lock().unwrap();
+                            if *a == 2 * i {
+                                *a += 1;
+                                break;
+                            }
+                        }
+                    }
+                    thread::current().id()
+                },
+                || {
+                    for i in 0..100 {
+                        loop {
+                            let mut a = m.lock().unwrap();
+                            if *a == 2 * i + 1 {
+                                *a += 1;
+                                break;
+                            }
+                        }
+                    }
+                    thread::current().id()
+                },
+            )
+        });
+
+        assert!(r1 != r2);
+    }
 }