States of matter are determined by temperature. When the temperature is more cold, the particles move less. The colder the temperature, the more solid something becomes. Because the particles are in fixed positions, they do not move much, they are colder, and they are solid. Also, you cannot pass your hand through locked atoms. If you swipe your hand through water or gas, it parts. Solids, however, are fixed and cannot be passed by without significant force to break it.
