Photosynthesis is the process by which green plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen, using energy from sunlight. This glucose serves as food for the plant, while the oxygen is released into the atmosphere.

The human brain is responsible for a wide range of functions including processing sensory information, controlling movement, regulating bodily functions, and enabling learning and memory. It's a highly complex organ that plays a crucial role in our daily activities.

The theory of relativity, developed by Albert Einstein, consists of two main ideas: special relativity and general relativity. Special relativity deals with objects moving at constant velocity, while general relativity extends these ideas to account for gravity. Both theories have had a profound impact on our understanding of the universe.

Vaccines work by stimulating the immune system to produce an immune response against a specific pathogen, such as a virus or bacteria. This trains the immune system to recognize and fight the pathogen if the person is exposed to it in the future, providing immunity to the disease.