When I put flowers in the vase and insert the stalk into the floral foam, the weight of the flower itself will unbalance the vase. Then I pour some water into the vase and see what happens: The vase stands up independently with flower inside!

Because of the weight of water, the gravity center becomes lower so that the vase can maintain balance again. In another words, the vase is balanced when it has enough water, it will be tilted to one side when the water is required. Thus, people could be told to water the flower by the unbalanced vase.