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!