Bacteria were the first microorganisms that people actually observed with their own eyes using a microscope. Before that, people used simple lenses, such as magnifying glasses, to take a peek at larger objects, such as insects. However, in 1676, Antonie van Leeuwenhoek turned the world upside down by observing bacteria in a sample of water. He chose to use high-quality single-lens microscopes instead of the early compound microscopes, which were a bit crude in terms of their resolution at the time. However, his findings laid the foundation for all future work done using compound microscopes.