Edited June 8th:
Now that "How to Change Triangular Numbers into Square Numbers" has been published, here is the link.
Here is a triangle made of four rows of pennies.
This image shows a pair of figures mapped to a third shape.
If you had not read the captions, would you see the relationship between the second and third images?
Writing Tips:
Create your own images when necessary. The article will have another image found on the web. I simply could not imagine how to search for anything like the last two.
Even as simple a tool as Microsoft Paint crops and annotates images taken with a digital camera. If I had remembered the "crop" feature, I could have taken less fuzzy images in the first place.