Some years ago when my oldest child was learning about "buoyancy' in her science class she was required to do some simple experiments in the kitchen using different types of objects to see how well or if they floated.  At the same time she was also doing experiments that demonstrated "surface tension."  We had fun doing these experiments together and talking about them.  It was at this time that we came up with the idea for our "Sinking Self" jar.  The purpose of the jar was to encourage good behavior and respect in the home.  I used a gallon jar which the kids decorated with different words expressing obedient and respectful responses like, "Yes, Mom", "Okay", "Yes, Dad", etc...  We filled the jar about 3/4 the way full with water and put a few drops of blue food coloring in it to make the water easier to see and floated an empty, clear plastic cup on top of the water.  Then, as I noticed times when one of my children suppressed self to obey Dad or me or share with a sibling or do something to help another, etc... I would tell them to put a marble in the plastic cup.  As the plastic cup got fuller and fuller, of course, it began to sink deeper and deeper in the water until the final marble would send it to the bottom of the jar.  At this point there was much rejoicing because we had already set up a reward to celebrate the sinking of the "self" cup; going to the zoo, Childrens Museum etc...  We used the "Sinking Self" jar for quite a few years.  It is a great visual motivator for young children.  I think it's time to resurface it for my second wave of youngin's!
(Just a note:  The three cups around the large jar each contained a different color of marbles for each child.  They liked to take notice of how many they had contributed to the "self" cup.  What's interesting is that my more "strong willed" child would often times have more marbles in the "self" cup, probably because he was also the most competitive.)