So, I started with a $10$ by $10$
green square,
I marked the half-way point along
the top edge,
I then drew a line from that point
to a corner at the base.
Then I cut the corner piece out
along that line and made a change of colour, so I ended up with
these two shapes:
What can you say about these two
shapes?
Can you say something about the
relationship between them?
Do you have any other ideas about
these two shapes?
What about starting with a rectangle like this ...
Then you could mark it at half-way or elsewhere ...
What can you say now?