Attracted, went on the links, from the discussion on the Hebra pulled out this charm: "It is not necessary to say in one class that it is not possible to do this, and in the next to tell what you can, but with conditions.
It is not necessary to immerse the child immediately in the whole field of mathematics, but the current teaching reminds us of the following, if it were about geography:
Class 1 - the earth is flat and there is only one our country, the sun revolves around the earth.
Class 2 - the earth is flat, but there are other countries on the same continent, the sun revolves around the earth
Class 3 - the earth is flat, there are other continents and countries, the sun revolves around the earth.
Class 4 - the earth is curved, there are other continents, countries and peoples, the sun revolves around the earth.
5th grade - the earth is round, and what to spin around will tell you on the lesson of astronomy from 6th grade.