Take a sponge soak it in a water and flower solution. Then roll the sponge real tight and tie a peice of string or twine around it. Wait for the sponge to dry and then go find a toilet. Once you find a toilet you cut the string and push down on the flush lever. During the process of flushing, throw the sponge into the toilet. The sponge will get into the pipe and expand, cuasing the toilet to back up.

You can substitute the flower and water solution with hairspray.