Thinner plastic allows the soil to heat up better but also tears more easily. Use plastic with a thickness of 1 to 4 mils for best results. Fill in the ditch around the perimeter with soil to hold the plastic in place. You may also wish to add rocks to the edges to help hold down the plastic.
The didgeridoo is a wind instrument traditionally made from the trunk of termite hollowed eucalyptus trees from Australia (like the one pictured at left, which is being played by our mate Lewis Burns).
The sound waves vibrate around inside the didgeridoo and there you go, you get sound! Sorry 'bout my not so good explaining, I hope you understand:) A few times my school has had excursions or visitors about aboriginal culture. A few times we learned about didgeridoos.