diff --git a/tests/StringTest.php b/tests/StringTest.php
index 6304c60..21f9a2b 100644
--- a/tests/StringTest.php
+++ b/tests/StringTest.php
@@ -16,3 +16,11 @@
     $this->assertEquals('foo_bar', Str::snake('fooBar'));
     $this->assertEquals('foo_bar', Str::snake('fooBar', '_'));
 });
+
+it('can convert string to lower case', function () {
+    $this->assertEquals('foo bar baz', Str::lower('FOO BAR BAZ'));
+});
+
+it('can replace string', function () {
+    $this->assertEquals('bar bar baz', Str::replace('foo', 'bar', 'foo bar baz'));
+});