Complex sentence: This barn explodes because when Marley throws a knife at Thornton the knife hits a barrel filled with 1203 and when Thornton shoots at Marley, the bullet hits the 1203 and the barn explodes.
Split into simpler sentences: This barn explodes because when Marley throws a knife at Thornton the knife hits a barrel filled with 1203. When Thornton shoots at Marley, the bullet hits the 1203, and the barn explodes.