diff --git a/ch11/adder/Cargo.lock b/ch11/adder/Cargo.lock
new file mode 100644
index 0000000..efe4e14
--- /dev/null
+++ b/ch11/adder/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "adder"
+version = "0.1.0"
diff --git a/ch11/adder/Cargo.toml b/ch11/adder/Cargo.toml
new file mode 100644
index 0000000..9a5826a
--- /dev/null
+++ b/ch11/adder/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "adder"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/ch11/adder/src/lib.rs b/ch11/adder/src/lib.rs
new file mode 100644
index 0000000..818a362
--- /dev/null
+++ b/ch11/adder/src/lib.rs
@@ -0,0 +1,31 @@
+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}
+
+#[derive(Debug)]
+struct Rectangle {
+    width: u32,
+    height: u32,
+}
+
+impl Rectangle {
+    fn can_hold(&self, other: &Rectangle) -> bool {
+        self.width >= other.width && self.height >= other.height
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn explore() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+
+    #[test]
+    fn another() {
+        panic!("Fail!")
+    }
+}