Our ideas about cells have evolved over time as new technologies have emerged, and ideas once thought to be true  were revised to reflect more accurate information. As modern science has advanced, scientists have sought to prove or disprove what was once considered common knowledge, so that ideas began to be supported by tests repeated over and over to prove the same result.
One of the outcomes of this process has been the development of  cell theory—a collection** of ideas and conclusions developed over time by many different scientists who combined new information with past knowledge to describe cells and how they organize and operate.
If you were to ask a scientist today what a cell is, there is a good chance you would be told they are the smallest unit of structure and function, but what does this actually mean? To paint a clearer picture, let’s go back to 1665, when and English scientist, Robert Hooke, became the first person to systematically view and describe cells.
While examining a slice of cork using an early version of the microscope, Hooke noticed tiny boxes  whose size he thought comparable to that of a small room. This led him to call them cells. Prior to his discovery, though scientists knew some things were living, they lacked the technology to know with certainty of what living things were made.
People soon began to work on inventing more sophisticated microscopes, and in 1674, a Dutchman named Anton van Leeuwenhoek used a powerful magnifying lens he created to examine pond water. Anton was surprised to see tiny organisms in the fluid, and upon examining additional samples, observed single-celled organisms he called animalcules.
This was the first time scientists could see microscopic living things, and the breakthrough significantly increased scientific understanding about how cells live and grow. Scientists now view living organisms as having certain traits in common. Generally speaking, they regard something as living if it takes in nutrition, reproduces, uses energy, grows, and responds or adapts to the surrounding environment.
A cell is the smallest part of a living entity that can do all these things, and though a living organism can consist of just a single cell, such as an amoeba, many consist of  trillions, which include the fish, birds, reptiles, amphibians, mammals and vegetation with which most people are more familiar. Regardless of size, however, all organisms reproduce and grow in the same way—as old cells are replenished by the formation of new ones.
In brief, cell theory tells us three important things about cells: all living things are made up of cells, a cell is the smallest unit in a living  thing; all cells come from other cells.