As T decreases the molecules slow, it becomes easier to attach to each other (with less energy they don't bounce off as quickly) and at some point they slow to the point that liquid forms. As T decreases further the molecules of liquid lose even more energy and at some point they form a crystal lattice (a solid). This is sketchy but you can add to it.
Because they are to tightly packed together
The solids are tightly arranged and have high attractive forces between them. These attractive forces do not allow the gas particles to escape from one another.
I believe this is how frost is made and condensation
