GAPS between the particles are TINY and hence it is tough to COMPRESS them.Solid has a fixed shape and volume.Properties of Liquid -In a liquid, particles are less TIGHTLY packed as compared to solids and less difficult to compress as compare to solid because particles have less space between them to MOVE but more than the that of solid.Liquids take the shape of the container in which they are kept.Properties of Gas -In gases, particles are far apart from each other, the force of attraction between the particles is negligible and they can move freely.The gaseous state has the highest compressibility as compared to solids and liquids.Gases have neither a fixed volume nor a fixed shape.So, A substance has a definite shape as well as definite volume. This substance is Solid.