The brain controls everything we think, feel and do, but it is very hard to study. It is made of billions of neurones with a huge number of connections, and different regions work together on most tasks. It is also protected by the skull, so scientists cannot simply look inside a healthy, working human brain or test it with the kind of experiments used on other organs.
One source of evidence is the case study of a person whose brain has been damaged by injury, stroke or disease. If a patient loses a skill after damage to one region, that suggests the region is involved in that skill. But case studies are difficult to obtain, because each injury is rare and different. They are also difficult to interpret. Damage often spreads over several regions, scientists usually do not know how the person's brain worked before the injury, and one person may not represent everyone.
There are also ethical issues, which are questions about what is right and wrong. Experiments that could harm a person's brain cannot be done on human volunteers. Patients may be unable to give consent if their injury affects their thinking, and their medical details must be kept private. Research on animals avoids some of these problems but raises its own concerns about animal suffering, and an animal brain is not identical to a human brain.