Calling the failed function: 'should not be called statically' in laravel 5.4

I use version laravel 5.4

In Helpers/Helper.php

public function test()
{
  return 'success';
}

In controller

use App\Helpers\Helper;

public function index()
{
  $val = Helper::test();
  dd($val);
}

Error : Helper::test() should not be called statically

I called the function inside helper to use it. But got the error as above. Please show me how to get the function in helper. Thank you

via Chebli Mohamed