Mice are some of the most common household pests, but it's not always clear how they're getting into your home. Mice usually start appearing once the outside temperatures fall and they begin to seek a warm place to shelter over the winter months. They usually enter homes through cracks and holes in walls, floors and foundations, though they can also come in through gaps in windows or even through drain lines. Most homeowners do not recognize a mouse hole or potential mouse hole until there are signs of infestation. Because of the way that a mouse's body is shaped, it can fit through a hole that appears far too small for anything to crawl through.