The Story of Young Gauss


In the lesson of arithmetic, the teacher, who decided to do his own business, asked the students to compute the sum of all the numbers from 1 to 20.
Young Gauss right offered him the answer: the sum of the series is 210.

The teacher was very surprised when by the end of the lesson all the students gave their own versions of the task, which coincided with the answer of Gauss.

What was the idea of young Gauss?