Dirty grout lines between the tiles in your bathroom or kitchen can be a pain to clean. But all you have to do to keep them clean naturally is to use things you already have around the house. Pour hydrogen peroxide into a spray bottle and saturate the dirty grout. Let it sit for a few minutes, and use an old toothbrush to loosen the dirt, stains, mold or mildew. Wipe it up with a damp microfiber towel to reveal freshly clean grout. If you have stubborn stains, try a DIY paste made from white vinegar and baking soda, and use a brush to scrub away the grime.