Final answer:
To create a material that will crumble when the actor hits it, you could use potassium (K) as the answer suggests. Potassium is a highly reactive metal that reacts with moisture, causing the material to crumble.
Step-by-step explanation:
To create a material that will crumble when the actor hits it, you could use potassium (K) as the answer suggests. Potassium is a highly reactive metal that reacts vigorously with water and oxygen. When the actor punches the metal wall, the potassium will react with the moisture in the air or on the surface of the wall, causing it to crumble.