Mice are able to fit through small holes due to their small body size. Contrary to popular belief, mice do not have collapsible skeletons. A mouse's small size and flexible vertebrae are what allows it to fit into small holes and cracks.
According to Orkin, a trusted pest control service, mice enter homes and structures through holes and cracks that are found in flooring, exterior foundations and walls. Mice are extremely narrow around the middle, which allows them to slip into openings that are much smaller than their bodies.
Typically, mice infestations occur during colder periods, as the mice seek proper shelter and food they cannot find outdoors. They tend to infest homes in colonies, meaning once one mouse is spotted, there are more where it came from. Mice reproduce rapidly once they find shelter. It is not unheard of for colonies to become as large as 200 mice. Sealing up cracks and patching up holes with a resilient filler treatment helps prevent mice from entering homes. In the event of an infestation, it is always a good idea to consult with a professional exterminator, as they have several options that are safe and effective when it comes to eliminating rodents.