What to use as a target for Shuriken ninja stars & Kunai darts?

Just bought some ninja darts & ninja stars and cannot wait to throw them. But all of the wood that i have available at the moment is too hard for them to penetrate when you throw them at it. Even just laying the piece of wood on the ground and dropping it from a height will not stick into it. Is there anything else i can use as a target to practice throwing? Is there a certain type of wood that is perfect for knife throwing? Thank you for your answers.