Locking is not working

Locking is not working

This is my code:
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApplication4
{
class Writer {
public void Write(string xxx) { Console.Write(xxx); }
}
class Program
{
static Writer wrt;
static void Main(string[] args)
{
wrt = new Writer();
Thread trd = new Thread(new ThreadStart(delegate() {
lock (wrt)
{
Thread.Sleep(1000);
wrt.Write("1");
}
}));
trd.Start();
wrt.Write("0");
Console.ReadLine();
}
}
}
The excepted output is "10", but the output is "01". Why?