Bricks or blocks, however you call it, are a common, cheap, and very useful building material made of baked clay.
Blocks are one of man's oldest permanent building materials.
Bricks are durable, easy to build into walls, and cost little to make.
Ordinary brick can stand the direct flames of a fire with little damage, and firebrick (a special brick used for lining furnaces) can stand temperatures as high as 4000 degrees Fahrenheit.
The clay in brick is highly resistant to acids, so that brick walls can withstand the smoky, corrosive air of cities better than some kinds of stone and painted metal.
Bricks have been made in many sizes and shapes since they were first used over 5,000 years ago.
At the present time the standard brick in the United States is a rectangular block measuring about 21.
25 by 3.
75 by 8 inches.
Most bricks shrink a little during baking, so that few are exactly this size.
Bricks range in color from nearly white, through tan, red, and red-brown, to dark purple and blue.
The color is determined by the amount of iron and other impurities in the clay and by the method of baking, or firing, the brick.
Usually the higher the baking temperature used, the darker the brick will be.
Bricks may also be glazed in various colors, like pottery.
The strength of brick also varies a great deal.
Brick has high compressive strength; that is, it can withstand forces that press in on it.
But brick does not have tensile strength -it cannot withstand forces that tend to pull it apart.
The average brick can take a load of about 5,000 pounds a square inch before it is crushed.
Several types of brick can stand a load as high as 10,000 pounds a square inch.