Cob is made from straw, sand and clay (this last ingredient can be handmade from water and earth if you are on a tight budget). You mix these ingredients together, making 'test bricks' which should stick together well and be hard when dry, but not brittle. Trial and error with different ratios is usually needed when starting your project, but eventually you'll create a test brick that is perfect, and can then begin mixing in larger quantities. Bricks of damp cob are then piled on top of each other and shaped to make the walls. The cob is stuck to a wooden frame, which supports the thatched roof.