Hi and thank you for your very interesting question!
Cells found in both animals and plants, or even in bacteria, are very complex systems comprised of smaller components which you can see in the diagram.
Imagine the cell being like a very big factory that has many different sites, each one of them responsible for a specific production process. For many years, it was believed that cells behave like flowing liquids, but recently it was shown that they behave more like solids. In fact, cells are comprised of a solid wall, called a membrane. This is filled with other liquid and solid-like components, such as the nucleus and mitochondria.
The main characteristic that a material needs in order to be characterized as a liquid is its ability to flow when you apply a force to it. Unlike liquids, cells don’t flow freely. They can adapt their original shape once a force is stopped applying them. So, it is more appropriate to consider cells as very soft solids – like a spring or a rubber band rather than very thick liquids – like honey or custard.
A very nice article with more detailed information on this can be found on the Institute for Bioengineering of Catalunya website.
Diagram: Internal structure of animal, plant, and bacteria cells.
Image credit: Encyclopædia Britannica, Inc.