So, I started with a $10$ by $10$
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
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?