If you notice a cracked or broken brick, or if the mortar between the bricks is crumbling, it might be possible to make the repairs yourself. Sometimes the hardest part about replacing a brick is finding one that matches the others. First, measure the existing bricks carefully to get the exact size, then chip off a piece of the broken brick and take it to a used brick dealer to find a substitute. If the brick you're replacing is under a window or other load-bearing part of the wall, it's best to call in a professional mason. If you decide to do the job yourself, the first thing to do is to remove the old brick with a hammer and chisel. Be sure to wear protective eyewear and heavy work gloves, at least on the hand that holds the chisel. Once the old brick is gone, use a dry paint brush to remove any remaining dust or chips, then moisten the cavity with water. Spread a layer of mortar on the bottom of the opening like you'd spread butter on bread. In fact, the process is called buttering. Do the same to the top and sides of the replacement brick and slide it carefully into the hole. Wait until the mortar is firm to the touch before finishing it with a pointing tool. For cracked mortar, remove the old mortar with a hammer and chisel, just like removing an old brick. Make an indentation at least an inch deep. Dampen the area slightly so the old mortar doesn't absorb water from the new mortar. Force mortar into the vertical cracks first, then the horizontal ones. Again, wait until it's dry enough that you can press your thumb into it and leave a print before finishing.