Who generates the endless WM_PAINT message

Hi,

I have a very program. In fact, it's the first example of Jeff
Prosise "Programming Windows with MFC" 2nd.
---------------------------- Header file
------------------------------
class CMyApp:public CWinApp
{
    public:
    virtual BOOL InitInstance();
};

class CMainWindow: public CFrameWnd
{
    public:
    CMainWindow();
    protected:
    afx_msg void OnPaint();
    DECLARE_MESSAGE_MAP()
};

---------------------------- CPP file -------------------------------
#include <afxwin.h>
#include "Hello.h"

CMyApp myApp;

BOOL CMyApp::InitInstance()
{
    m_pMainWnd = new CMainWindow;
    m_pMainWnd->ShowWindow(m_nCmdShow);
    m_pMainWnd->UpdateWindow();
    return TRUE;
}

CMainWindow::CMainWindow()
{
    Create(NULL, _T("Hello"));
}
BEGIN_MESSAGE_MAP(CMainWindow, CFrameWnd)
    ON_WM_PAINT()
END_MESSAGE_MAP()

void CMainWindow::OnPaint()
{
    static int n=0;
    TRACE("OnPaint %d\n", n++);
}

------------------------ END OF CODE
---------------------------------------

The OnPaint function is being called all the time, with the window not
being occluded,not cursor over it, etc. Can anybody tell me which
part of MFC generates the PAINT message?

Frank