Spontaneous Generation was the theory that life could spring from non-living materials. In the 1600's, people actually believed that mice came from straw and dirty laundry. People didn't have the microscopes to show fly eggs and cells. Francisco Redi proved this theory false using a very simple experiment.
In those days people believed that maggots (young flies) were born from rotting meat. Redi's experiment included meat, jars, and cheese cloth. He put meat into three jars for the experiment. One jar was left completely uncovered, as to show what happens when meat rots. The second jar was closed with a lid. Most people would say that the maggots obviously wouldn't grow because there was no air for them to breathe. This was the reason for the third jar in the experiment. He covered this one with cheesecloth. This way, air could go in. Maggots instantly grew in the open jar. Nothing was on the meat in the lidded jar. The cheesecloth jar had flies around it, but not in it. This proved the theory of spontaneous generation false.
Today there is no doubt that living things cannot arise from non-living things. Only living things can produce other living things.