What shape do you think is being made by joining the midpoints of the sides? Which properties of the shape do we have to establish to prove that it is this partilcar type of shape|?
In the diagram below, the diagonals $AC$ and $BD$ of the original shape have been drawn in.
What can you say about the lines $AC$ and $PS$?
What about lines $AC$ and $QR$?
Can you prove you conjecture?