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
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