The trick was to fill a plastic container of some sort with water and then put the container in the top of your toilet bowl. You may have to put in a few rocks to weigh the container down.
Why does this save water? This trick saves water and money because the water in the container takes up space in the bowl that new water would take up as the bowl when the toilet is flushed. You basically can make any toilet a low water usage toilet.
I don't know for sure how much money this saves but I figure the project was definitely a worthwhile one. The materials were free and the project took me about ten minutes for all the toilets in the house. If I can save a few bucks a month, that will be worth it to me!
Try this trick in you own house. If you are looking for ways to reduce costs, this is a very simple one to accomplish!