Ever have only a piece of clear or black plastic and want to use it as a shelter or dining fly? It's hard to do without grommets for tie downs, since holes cut in the tarp with lines tied through them soon tear out.
Try this technique - it will work with plastic sheeting, tarps, canvas, nylon, or just plain old lumber wraps. Even very light plastic painting tarps can hold up to quite a lot of wind when tied down this way.
Select a relatively round rock roughly the size of a golf ball. Lay it in the sheeting material near an edge you want to tie off or secure with twine or cord. Then using your cordage to tie off the sheeting material, make a simple overhand slip knot.
If you put a simple overhand jam-knot in the end of the cordage before making the overhand slip knot, when the slip knot is pulled tight, it can't slip past the jam knot you've made. As illustrated, when the cord is secured around the rock, encompassed by the sheeting material, it becomes a very secure holding point that won't slip or tear through.
Using this technique, even very light plastic sheeting becomes a workable shelter from the elements that is easily and securely tied down. Used lumber tarps, which are available free from most building supply centers, make a first rate dining fly when secured this way!