What causes knot on head when hit?

A knot on the head, often referred to as a bump, is caused by swelling due to trauma to the scalp. When the head is hit, blood vessels may break and lead to the accumulation of blood and fluid in the tissues, resulting in a localized swelling. This response is part of the body's inflammatory process to protect the injured area. In most cases, these bumps are harmless but can be painful and should be monitored for any signs of more serious injury.