Maximum and minimum of function

One day, my small brother came to me, and he asked me what is the maximum and the minimum of a function? Then I said "Hmm, do you know how to drow a function?" Then he said "Of course" Then I said "If you have a function and you draw it, you'll find a maximum point and a minimum point. Likne this picture.
Then he understood it.
He said that the mathematics is a beautiful world. Then I said " I totaly agree with you".