Here is what I say to my high school students:

 

Remember when you were a child and played with blocks? You put down two blocks, and then another on top. You then said,

 

“Wow! A house!”

 

You then put down another block.

 

“Wow! A corner!”

 

Well, the mathematician takes two ideas in his head, and gets a new idea.

 

“Wow!” he says, “A new idea!” 

 

This is what abstract thinking, such as mathematics, is about.