lock() question

[dannystommen]

I want to create a Singleton object. On MSDN and other places I found the same example like the one here:

public sealed class Singleton
{
    static Singleton _instance = null;
    static readonly object _lock = new object();

    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            lock (_lock)
            {
                if (_instance==null)
                {
                    _instance = new Singleton();
                }
                return _instance;
            }
        }
    }
}

It works, that's not the problem. But why is there a seperate object used for locking? Why not just use lock(_instance) instead of lock(_lock)?