this Instructable shows one how to make a fully functional hammock in under 5 minutes with household supplies.
Step 1: Materials
knife (Or anything that cuts)
10 feet of rope.
Step 2: Preparing the Sheet
cut the sheet in half lengthwise, or at about 2 1/2 ft from the edge if its not a twin.
Step 3: Tying the Rope.
cut the rope in half, and tie each piece to the ends of the sheet using the sheet-bend knot. http://www.animatedknots.com/sheetbend/index.php will show you how, if you don't know.
Step 4: Finishing
the last step: tie to a tree and relax.
Note- it may look incredibly thin, but it will widen when your in it.