Glass is made from three simple ingredients: sand, soda ash, and limestone. If sand is heated to its melting point, its properties change and create glass. By adding soda ash, the melting point is slightly lower, which makes the process more cost-efficient. Finally, limestone is added to make the material more stable—without the addition of limestone, the glass material would dissolve in water. Glass is a particularly interesting material because it is difficult to classify as a solid, liquid, or gas. Molecularly, it is closer to a liquid but appears solid. For this reason, it is sometimes called a frozen liquid.