diff --git a/src/Playground.jl b/src/Playground.jl
index c373e32..d82ac6a 100644
--- a/src/Playground.jl
+++ b/src/Playground.jl
@@ -4,4 +4,8 @@ export add_one
 
 include("add_one.jl")
 
+export double
+
+include("double.jl")
+
 end
\ No newline at end of file
diff --git a/src/double.jl b/src/double.jl
new file mode 100644
index 0000000..eea54a4
--- /dev/null
+++ b/src/double.jl
@@ -0,0 +1,3 @@
+function double(x)
+    return 2 * x
+end
\ No newline at end of file
diff --git a/test/runtests.jl b/test/runtests.jl
index f7d4c58..9305eb0 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -5,4 +5,10 @@ using Playground
     @test add_one(1) == 2
     @test add_one(0) == 1
     @test add_one(-1) == 0
+end
+
+@testset "double" begin
+    @test double(1) == 2
+    @test double(2) == 4
+    @test double(-2) == -4
 end
\ No newline at end of file