diff --git a/src/spline.rs b/src/spline.rs
index c7b7749..5a6c4db 100644
--- a/src/spline.rs
+++ b/src/spline.rs
@@ -28,12 +28,17 @@ use crate::key::Key;
 pub struct Spline<T, V>(pub(crate) Vec<Key<T, V>>);
 
 impl<T, V> Spline<T, V> {
+  /// Internal sort to ensure invariant of sorting keys is valid.
+  fn internal_sort(&mut self) where T: PartialOrd {
+    self.0.sort_by(|k0, k1| k0.t.partial_cmp(&k1.t).unwrap_or(Ordering::Less));
+  }
+
   /// Create a new spline out of keys. The keys don’t have to be sorted even though it’s recommended
   /// to provide ascending sorted ones (for performance purposes).
-  pub fn from_vec(mut keys: Vec<Key<T, V>>) -> Self where T: PartialOrd {
-    keys.sort_by(|k0, k1| k0.t.partial_cmp(&k1.t).unwrap_or(Ordering::Less));
-
-    Spline(keys)
+  pub fn from_vec(keys: Vec<Key<T, V>>) -> Self where T: PartialOrd {
+    let mut spline = Spline(keys);
+    spline.internal_sort();
+    spline
   }
 
   /// Create a new spline by consuming an `Iterater<Item = Key<T>>`. They keys don’t have to be
@@ -159,6 +164,12 @@ impl<T, V> Spline<T, V> {
     })
   }
 
+  /// Add a key into the spline.
+  pub fn add(&mut self, key: Key<T, V>) where T: PartialOrd {
+    self.0.push(key);
+    self.internal_sort();
+  }
+
   /// Remove a key from the spline.
   pub fn remove(&mut self, index: usize) -> Option<Key<T, V>> {
     if index >= self.0.len() {
diff --git a/tests/mod.rs b/tests/mod.rs
index f66f383..fbbdcff 100644
--- a/tests/mod.rs
+++ b/tests/mod.rs
@@ -173,6 +173,30 @@ fn nalgebra_vector_interpolation() {
   assert_eq!(Interpolate::lerp(start, end, 0.5), mid);
 }
 
+#[test]
+fn add_key_empty() {
+  let mut spline: Spline<f32, f32> = Spline::from_vec(vec![]);
+  spline.add(Key::new(0., 0., Interpolation::Linear));
+
+  assert_eq!(spline.keys(), &[Key::new(0., 0., Interpolation::Linear)]);
+}
+
+#[test]
+fn add_key() {
+  let start = Key::new(0., 0., Interpolation::Step(0.5));
+  let k1 = Key::new(1., 5., Interpolation::Linear);
+  let k2 = Key::new(2., 0., Interpolation::Step(0.1));
+  let k3 = Key::new(3., 1., Interpolation::Linear);
+  let k4 = Key::new(10., 2., Interpolation::Linear);
+  let end = Key::new(11., 4., Interpolation::default());
+  let new = Key::new(2.4, 40., Interpolation::Linear);
+  let mut spline = Spline::from_vec(vec![start, k1, k2.clone(), k3, k4, end]);
+
+  assert_eq!(spline.keys(), &[start, k1, k2, k3, k4, end]);
+  spline.add(new);
+  assert_eq!(spline.keys(), &[start, k1, k2, new, k3, k4, end]);
+}
+
 #[test]
 fn remove_element_empty() {
   let mut spline: Spline<f32, f32> = Spline::from_vec(vec![]);