I am intentionally avoiding the standard term which, by the way, did not exist in Euler's time. One of the ugliest outgrowths of the "new math" was the premature introduction of technical terms.

A mathematics teacher is a midwife to ideas.

Analogy pervades all our thinking, our everyday speech and our trivial conclusions as well as artistic ways of expression and the highest scientific achievements.

There are many questions which fools can ask that wise men cannot answer.

There exist a lot of questions that the fools can ask, and the intelligent cannot answer.

The elegance of a mathematical theorem is directly proportional to the number of independent ideas one can see in the theorem and inversely proportional to the effort it takes to see them.

If you cannot solve the proposed problem try to solve first some related problem.