This was the case for quite a long time, and everything was fine. Until in the second decade of the XX century, out of nowhere, a Special Theory of Relativity suddenly appeared which came into conflict with the concept of "mass" that existed at that time. After all, according to the Special Theory of Relativity, the mass of a body functionally depends on the speed. But if the mass is the amount of matter, then this very amount of matter cannot depend on the speed. According to the Special Theory of Relativity, it turned out that the higher the speed, the more the amount of matter became, and if the speed decreased, the amount of matter decreased. This immediately, immediately made the Special Theory of Relativity untenable. There is a paradox, since the theory has come into conflict with the existing concept of "mass". At the same time, Einstein himself did not notice this paradox, or pretended not to notice it, and did not revise the definition of mass in any way. Mass, well, mass, just began to make formulas according to which mass depended on speed. If Einstein himself had noticed that the concept of mass contradicts his theory and would have started to revise this concept himself, it is still unknown how the fate of his theory would have developed. At that time, Newton's authority was still unquestionable. Therefore, for a long time, the paradox we have described calmly existed and mass was calmly the amount of matter, and this amount of matter was functionally dependent on speed. However, as time went on, this paradox began to stick out, and students began to ask unpleasant questions. At that time, students asked more questions if they did not understand something-this is a clear sign that the students understood something. Now students don't ask questions – a clear sign that they don't understand anything, just like their teachers. So, in order to remove this contradiction, the followers and epigones of Einstein decided not to declare special relativity untenable, but to declare mass by an indefinite term – "physical quantity". Here, the mass already loses its independent material content and becomes a calculated value, as it should be according to the special theory of relativity. Therefore, already in 1969, the amount of matter or matter disappears completely from the definition of mass. In the sixth grade, the concept of mass is not considered at all, and in the seventh grade, the definition of mass is as follows: