.net Lock - Two questions

Two questions about Lock () in .net

Firstly, I know that if an object is locked inside one class and another class tries to lock the same object, this causes a dead end. But why? I read about it on MSDN, but MSDN is rarely all clear.

---- Edit Question One ---- Still confused. I have a main thread (UI thread) that generates many Threadpool threads. Each child stream blocks data before it works with it. It works great every time.

If I then try to block the same data from the user interface thread to check if I should even worry about creating a new thread for the edge case, I create a deadlock almost every time.

---- Change the second question ---- Secondly, if I have a complex object that I am blocking, are all child objects inside it locked too? Demo version:

internal sealed class Update
{
    //Three objects instantiated via other external assemblies
    public DataObject One { get; set; }
    public DataObject Two { get; set; }
    public ReplayStatus Status { get; set; }
}

If I call lock (UpdateObject), are each of the three internal objects and all child objects also locked?

Therefore, I should do something like this to prevent playback streams with my data objects:

Lock(UpdateObject.One)
{
    Lock(UpdateObject.Two)
    {
        Lock(UpdateObject.Status)
        {
            //Do Stuff
        }
    }
}