"Clinker brick" illustrated

The image above was submitted to the blackmagicfuckery subreddit by someone wondering why one brick in a sidewalk was not covered with the dusting of snow.  After dozens of inane replies ("Australian brick" "installed upside down, snow is on bottom") one knowledgeable Redditor provided the proper information:

This could be a brick called a 'clinker'

Clinkers are bricks that have different properties than normal bricks. They are used as decoration, paving and for water proofing buildings.

In the old days they fired bricks in a big kiln. All stacked on top of each other. They found that the bricks at the bottom experienced higher temperatures for longer. Turning them into a denser brick, closer to ceramic, that had a metallic "clink" sound when tapped with a hammer or another brick.

For a time these clinkers were not wanted because they have a high thermal conductivity, meaning they transport heat and cold into/out of your house better, that's bad. Then someone figured out they make great road pavers. Being harder than normal bricks they take longer to wear out.

Some people used them as building decorations because they are usually a darker colour than normal bricks. And some people realised that they are waterproof and started using them as the outside layer in double brick buildings. With increased demand they started to purposefully make clinkers for decoration, waterproofing and road paving.

Looks like magic, but it's just science.  You learn something every day.