At the top you see a bar which contains a variety of different objects. Scroll through the objects using the arrows on the sides of the bar.
Click or drag the object to drop the object in the water. The object will either float on top of the water or sink to the bottom of the tank. You can add as many objects to the water as you like.
At the bottom right, there is a “reset”-button. Use this to remove all objects from the tub and start again. At the top left there is a sound-button. Use this to turn the audio of this tool on or off.