Before we go further, please watch this
is also interesting.
Note that splitting a brick has a trick to it. Before hitting it down you are supposed to lift it slightly over a hard edge. You hit the brick, and it starts moving down towards the hard edge. The hard edge (or just edge of whatever the brick is on) serves as a tool just as a hammer would.
Most people should be able to split a brick with a hammer. Imagine using a stationary hammer to smash a brick.
Now imagine the brick and the hammer only half an inch apart. Gain enough momentum with your arm, transfer the energy to the brick (ouch?), and voila!
I'm not saying anyone can do it, it needs strength and skill. just as ripping a thick soft cover book apart.
Now ends my boring rant on brick physics :P