Though most cracks start small, they grow over time. Eventually, water seeps in and the grout begins to crumble. To repair grout, you'll need a few inexpensive specialty tools, including a toothed tool called a grout saw (available at tile dealers), a sponge, and a rubber grout float. You'll also need to buy replacement grout that is the same color.

1) Use a grout saw or the pointed end of a lever can opener to remove cracked and loose grout and to score stable grout. Brush away the dust and vacuum the surface.

2) Mix replacement grout according to the label directions.

3) Holding the rubber grout float's leading edge up at a slight angle, spread the grout across the surface. Be sure to work it into all joints completely.

4) Use your finger to smooth the grout at the joints.

5) Allow the grout to set up for about a half hour and then use a damp sponge to wipe the excess off of the tile surface.

6) Let the grout dry for several days and then apply penetrating silicone sealer for protection.

Note: If you have a newly tiled interior floor and the grout is cracking, consider removing the grout between tiles as discussed above, and replacing it with a sanded caulk. This only works if grout joints are narrower than 1/4 inch because the caulk is not designed for wider gaps. Because the cracking is probably caused by floor deflection from foot traffic, consider trying to reinforce the floor from below to strengthen it.