Having a mice infestation is a big problem in any home, but there is a way to prevent the rodents fromcoming in. Mice can fit into even small holes in the foundation and in gaps in the siding of a house, so proper identification of these trouble spots is key. Once you have found a potential area for a mouse to enter the building, using steel wool is a simple, cheap and safe way to keep the mouse out.
Steel Wool Type
Steel wool makes an excellent barrier to mice infestations because they cannot chew through it. It's simply too strong for even larger mice and as such makes a much better gap-filling material than traditional caulk or putty. You can use any kind of steel wool, as long as it is in fact steel and not a cheaper compound. Things such as plastic mesh may resemble steel wool but mice will quickly chew through it, rendering it useless as a barrier. The wool can be any thickness and weave type, as well. Tighter wool--wool with smaller gaps in it--will typically work better. Mice love to grab and tug on the steel wool if they can, and if the gaps are smaller, the chances are far less that they will pull the obstruction free. If large-gap wool is the only kind available it will still work, so long as you take more precautions securing it into place.
Using the Wool
Once you have identified the gaps or holes in your walls, insert a decent-sized piece of steel wool in the hole. Use enough wool so that it can fit all the way in but is still very snug and not easy to tug out. Then, using any brand of common household caulk, fill the hole around the steel wool. This will harden and help keep the wool in place, preventing even the most pesky of mice from dragging it free.
If you use proper steel wool, there should be no danger of the mice eating it. However, larger mice might be able to chew off pieces of more fragile or thinner steel wool. If this happens, it will almost certainly lead to death for the mouse, as the pieces of steel will cause internal bleeding as it is digested. If you encounter a rotting smell, check the steel wool for any signs of chewing or missing pieces. If you find some, chances are you have a dead mouse nearby that will need disposing of. Prevent this problem with a solid layer of caulk over the steel wool, to keep the mice away from it.