the size of the bathtub will be change according to the user , to save more water, 

we don't need to fill the bathtub with water if a kid would like to have a bath !!!!!! 

This bathtub could be used in hotels for examples

Example :

In Dubai there are 74000 rooms and that mean 74000 bathtub, if we save 10 liters of water every day 74000*5=740000 liter daily in one city (Es klingt gut. Oder ? )

It might be expensive to have one like this, but it is not only about saving money, it is luxury too