What are cells made of? Biologists have been studying cells since the mid-1600s, when Robert Hooke viewed a slice of cork through a microscope and coined the word "cell" to describe the walled-in spaces he saw. Most cells are invisible to the naked eye. Yet they carry out the many complex processes that make life possible. As microscopes have improved, scientists have learned more and more about cells and their organelles-the structures within cells....

How are polar bears related to pandas? For thousands of years, philosophers and scientists have tried to organize and understand, or classify, the relationships among Earth's animals and plants. Early classification systems were cumbersome and inconsistent. In the late 1720s, Carl Linnaeus began developing a classification system to describe relationships among all living things, including animals, plants, fungi, and bacteria. This organization, called...

Outraged people claimed that Darwin's theory had made humans the relatives of monkeys. Scientists were sure that species changed over time, but no one could explain how. In the 1800s, Charles Darwin's studies of thousands of specimens of living things showed that no two individuals of any species were exactly alike. He realized that over millions of years, some individuals had traits that gave them an edge to survive and reproduce. As they reproduced,...

Since prehistoric times, people have wondered what causes disease. Early people blamed evil spirits. Later, disease was thought to be caused by an imbalance of bodily fluids. By trial and error, people discovered plants that cured certain ailments. But disease still spread through dirty, crowded cities. In 1546 an Italian physician proposed that tiny, invisible bodies cause disease. By the end of the nineteenth century, doctors had discovered the...

Since prehistoric times, people have wondered how the universe works. Early scientists studied how forces affect objects and watched how heavenly bodies move. In 1687 Isaac Newton published a set of laws that described the motion of all objects, both on Earth and in the heavens. By 1900 many physicists believed only a few questions remained to be answered. But the early 1900s brought revolutionary developments in physics. One was Albert Einstein's...