Telling a kid a secret he can findout himself is not only bad teaching, it is a crime.

Geometry between the Devil and the Deep Sea
Author(s): Hans Freudenthal
Source: Educational Studies in Mathematics, Vol. 3, No. 3/4, Lectures of the Comprehensive
School Mathematics Project (CSMP). Conference on the Teaching of Geometry (Jun., 1971), pp.
413-435
Published by: Springer
Stable URL: http://www.jstor.org/stable/3482035 .