For hundreds of years before the seventeenth century, thinkers explained that sudden appearances of life, such as mold on food or moss on rocks, were caused by "spontaneous generation,” which is the idea that life frequently springs up from non-living material. Thanks to the use of scientific experiments, which required repeatable tests, and the invention of the microscope, scientists such as Louis Pasteur discovered that these appearances of life were actually caused by microscopic organisms in the air that could stick to surfaces and grow into large populations we can see with our eyes.