Why are solids solid gases gaseous and liquids liquid?

States of matter (and that's what is at issue here) have to do with atomic or molecular cohesion in the materials being inspected. If the kinetic energy of the group is high enough, atom-to-atom bonds (or molecule-to-molecule bonds) fail and the units of the material will spin, vibrate and in general disco dance away from any partners into the next "upper" state of matter. A solid lead bar holds its shape. At sufficient temperature, the atom-to-atom bonds (the metallic crystal bonds) that maintain the bar's shape fail and the structure falls apart turning into a liquid. A different type of "stickiness" takes place to give the liquid some surface tension, but it will take the shape of its container. (No more bar.) Water (which has taken the shape of its container) will, when heated sufficiently, gain so much energy that the molecules will escape the molecule-to-molecule bonds of the liquid and become a vapor that transitions into the gas we call steam. There are exceptions to the "rule" like when dry ice (solid carbon dioxide) changes directly to a gas (called sublimation) without passing through a liquid phase. There are a range of different "stickiness methods" for materials depending on the temperature and the nature of the atoms/molecules that make up that material. States of matter are a general set of ideas, and are a great place to start when thinking about the macroscopic structure of a stuff.