You are very fortunate to have been born when you were. Throughout most of human history, scientists knew very little about life and how it works. It was not until 1665 that a scientist by the name of Robert Hooke first discovered the existence of cells.
While using the newly invented compound microscope to look at a thin slice of cork, Hooke saw tiny room-like structures that he named cells. The cells that Mr. Hooke saw through his microscope were all dead, because the cork came from a plant that had long since died.
Nearby, a Dutch scientist by the name of Anton Van Leeuwenhoek was also using his microscope to look at things. Anton Van Leeuwenhoek decided to try and look at things that were still alive, like blood, and saliva from his mouth. Mr. Leeuwenhoek also looked at rainwater through his microscope.