diff --git a/mix.exs b/mix.exs
index 9c76235..a888b18 100644
--- a/mix.exs
+++ b/mix.exs
@@ -23,6 +23,7 @@ defmodule Aoc.MixProject do
     [
       # {:dep_from_hexpm, "~> 0.3.0"},
       # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+      {:mix_test_watch, "~> 1.0", only: [:dev, :test], runtime: false}
     ]
   end
 end
diff --git a/mix.lock b/mix.lock
new file mode 100644
index 0000000..df6ed49
--- /dev/null
+++ b/mix.lock
@@ -0,0 +1,4 @@
+%{
+  "file_system": {:hex, :file_system, "0.2.10", "fb082005a9cd1711c05b5248710f8826b02d7d1784e7c3451f9c1231d4fc162d", [:mix], [], "hexpm", "41195edbfb562a593726eda3b3e8b103a309b733ad25f3d642ba49696bf715dc"},
+  "mix_test_watch": {:hex, :mix_test_watch, "1.1.1", "eee6fc570d77ad6851c7bc08de420a47fd1e449ef5ccfa6a77ef68b72e7e51ad", [:mix], [{:file_system, "~> 0.2.1 or ~> 0.3", [hex: :file_system, repo: "hexpm", optional: false]}], "hexpm", "f82262b54dee533467021723892e15c3267349849f1f737526523ecba4e6baae"},
+}