Theory:

What are cells?
Living organisms perform certain essential functions such as respiration, digestion and excretion for their survival. All these functions are performed by sets of organs which are formed by structural units known as cells.
The smallest structural and functional unit of living organisms is known as a cell.
Cells vs bricks
Cells
Bricks
Cells are living structures. Buildings are non-living structures.
These are the structural and functional units of living organisms.These are the structural units of buildings.
Cells are organised to make the body of every organism.Bricks are organized to construct buildings.
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           Cells observed in an onion peel
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Brick wall
 
Important!
The egg of a hen is a single cell which can be seen by the naked eye.
 
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Discovery of cell:
The cells are extremely small in size and can be usually seen only under a microscope of high magnification. Robert Hooke first discovered cells in 1665.
Hooke examined a thin slice of cork under the microscope in 1665. During this, he noticed that the cork had partitioned boxes or compartments that appeared like a honey-comb.
 
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Cork cells under microscope
 
He also observed that each box was separated from the other by a partition or a wall and named these boxes as cells.
The cells that were observed by Robert Hooke were actually dead cells.
Even though cells were discovered in 1665, a very little was known about the cell for the next 150 years after Robert Hooke’s observations. But today a lot is known about the cell structure and its functions because of the inventions of microscopes with high magnifications.
Cell theory:
The cell theory was proposed by two German scientists called Matthias Schleiden and Theodor Schwann.
The postulates of the theory are as follows:
  • All living organisms are made up of cells.
  • The cells are the basic structural and functional units of living organisms.
  • All the new cells arise from the pre-existing cells.
Reference:
https://commons.wikimedia.org/wiki/File:Robert_Hooke,_Micrographia,_cork._Wellcome_M0010579.jpg