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If you slice a cone at a diagonal, you get an ellipse:
 


Here's what geometrically makes an ellipse an ellipse:

You start with the two green dots...

 

So, if you put two nails on a board and put a loop of string around them...

You can draw a perfect ellipse! Just keep the string pulled tightly.

Think about it... That loop of string will always be the same length -- won't it?

That's what makes an ellipse!