Can you explain with a little more details?
I believe that the phrase from Mandela's speech that is an example of a metaphor is the first one - 'the children, [...] the greatest of our treasures."
A metaphor is a type of comparison, where two or more things are compared without using words such as <em>like </em>or <em>as. </em>Here, the children are the greatest of our treasures - they are compared to a treasure, something which is the most important thing of all.
