Threading - Make a thread sleep until a worker finishes

I want to make the main thread in my app sleep while I have a worker
do some arbitrary task. The problem is waking the main thread back
up. Can anyone make this code sample work?

private void button1_Click(object sender, System.EventArgs e)
{
//save ref to main thread
th = Thread.CurrentThread;

Thread worker = new Thread(new ThreadStart(GetString));

worker.Start();
//go to sleep
Thread.Sleep(Timeout.Infinite);

MessageBox.Show(sData);

}
private void GetString()
{
//do something arbitrary on the worker thread
sData = "Something Pointless";

//go to sleep just to make sure
//it calls interrupt after main thread is asleep
Thread.Sleep(1000);

//wake main thread back up
th.Start(); //?? doesn't work, calling th.Interrupt() throws an
exception.
}