Use a pickaxe to break or kill mice and strange creatures, Or use a TNT to blow them up. Some of them just disappear/die and don't break. You can also kill them by pressing them with your mouse pointer. You will know that a mouse or creature has been killed because you will hear a Glass breaking sound and you will see the pickaxe turning/spinning to bash them in the head! eek! This game was only made for fun, especially for cats because cats don't like mice and they want to attack them.
I learned all this, I did the mice coding all by myself! :)