Calling a static method from a class in another namespace in PHP

This code bellow gives me this error : Class 'MyNamespace\Database' not found. How do I reference a class that belongs to no namespace, from inside one ?

Class Database
{
    public function request()
    {
    }
}

namespace MyNamespace
{
    class MyClass
    {
        public function myFuction()
        {
            Database::request();
        }
    }
}