The objects will not always have the same mass because they might not be made of the same substance. Some substances are heavier or denser than others. For example; A plate made of clay will be lighter than one made out of gold or diamond. Also, if you fill a beaker with water and another with sand at the same level, you will notice that the beaker with sand will be heavier and weigh a lot more than the beaker with water. This is because sand is denser than water. Hope i helped. GOODLUCK.