Cells are the building blocks of all living organisms. Organisms made of a single cell, such as bacteria and the fungus yeast, are described as being unicellular.
Organisms made of many cells such as animals, most plants and many species of fungi, are described as being multicellular. Multicellular organisms can be made up of millions of cells.
Cells are microscopic, so to see their structure we need to use microscopes. We can see more structures clearly if we use stains to colour specimens before putting them under the microscope.
Stains are coloured dyes that are absorbed by some cell structures but not by others. An example of a stain which is used is iodine.
Watch this video to see the procedure for preparing an onion cell slide and examining it under a microscope.