Look at the drystone wall and you will see dark spaces between the stones.
Below is sketch of a brick wall; the gaps between the bricks are light , giving the impression of mortar
Now you can see the clear distinction between the two walls go out and explore the walls around you, or when out on a walk see how many of each wall type you can see. I hope you might use this opportunity to ‘see’ other every day things that you might have missed as you walked to school or played in the garden.