Tear pieces of cardboard into small pieces no bigger than 3-inches by 3-inches. Spray the cardboard with water to make it more pliable. Place the cardboard into a gallon plastic storage bag.
Blow up the balloon to form the rock base. Tie it closed. Set it aside.
Make a paper mache solution. Mix 2 cups of water to every one cup of flour. Stir the mixture until all of the lumps dissolve and the mixture is completely smooth. Add 1/2-cup of white craft glue. Stir it into the mixture.
Remove the cardboard from the plastic bag. One piece at a time, dip the cardboard in to the paper mache mixture and lay it in the balloon. Allow bumps to develop as the rock develops. Few stones are round and smooth. Cover the entire balloon. Allow it to dry and cover it again. Lay it on wax paper to dry.
Once the glue is thoroughly dry, lay the rock on top of newspaper. Spray the entire "rock" with black and gray faux stone spray paint. Lay it on wax paper to dry. After it dries, check it for missed spots and touch it up. The finished product resembles a large rock but remains light-weight.