You run to your girlfriends house and announce that you were running at a rate of 15 miles per hour. She knows this is not true because she knows

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.

because if the tool that you are using is not balanced it there would be false mass.