How do cells form a solid structure?
So how are the cells are arranged to form a solid structure a solid body?


Plant cells have cell walls, are filled with liquid, and have strong cell-cell adhesion. This gives a fairly rigid structure that may seem like a solid. Take a carrot for example, it seems solid. But if you put the carrot in a blender, you get carrot juice - not a solid. Also, when the carrot dries out, it loses its rigidity.

Trees

Woody plants contain a lot of cellulose, a water-insoluble polymer, outside of cells. This gives them even more of a solid (and dry) appearance, but the insides of their cells are aqueous as well.

Cells other than plant cells

There are cells that lack a cell wall, they are just surrounded by a lipid bilayer. Tissues made from these cells (muscle, brain, liver, etc.) don't seem solid at all. Animals do have connective tissue (mostly protein-based) and a skeleton that holds everything together unless there is trauma (predators, accidents, etc).


  How do cells form a solid structure?


Let me un-ask the question a bit. Sometimes they do form a solid-like structure, and sometimes they don't. Fibrous biological macromolecules play a large role in making tissues rigid, either directly or in combination with having cells filled with aqueous solution.