diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 6f4c130d8f318..a0270d747a9d5 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -1541,6 +1541,9 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Because the slices have to be the same length,
+    /// // we slice the source slice from four elements
+    /// // to two. It will panic if we don't do this.
     /// dst.clone_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);
@@ -1607,6 +1610,9 @@ impl<T> [T] {
     /// let src = [1, 2, 3, 4];
     /// let mut dst = [0, 0];
     ///
+    /// // Because the slices have to be the same length,
+    /// // we slice the source slice from four elements
+    /// // to two. It will panic if we don't do this.
     /// dst.copy_from_slice(&src[2..]);
     ///
     /// assert_eq!(src, [1, 2, 3, 4]);