Plasticine learns how to swim

Well done! Your answer is right!

The water level is lower than the mark on the glass (see picture below). The plasticine ball displaces less water than the hollow body out of plasticine.

The hollow body has a greater volume than the solid ball and therefore displaces more water. Our assumption has been confirmed: If a body can float or not is dependent on the amount of water it displaces. Even bodies that usually don't float, can "learn" how to float by increasing their volume.