Why do soilds have a fixed shape while liquids and gas do not

Because the molecules are so tightly packed together that they do not just "float" around

Well, b ecause the particles in liquids are very close together (barely further apart than in solids) liquids do not easily compress, so their volume is fixed. Gases can also flow, so occupy the shape of their whole container. They do not have a fixed shape ... Gases therefore can be compressed or expanded.