The human brain is the central organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sense organs, and making decisions as to the instructions sent to the rest of the body. The brain is contained in, and protected by, the skull bones of the head.
Given this paragraph about the human brain, how does my brain know that I'm touching something hot?
Humans process information from the various sensory systems via the central nervous system. The nerves in your hand are integral to your sense of touch. When your fingers touch something hot, the nerves in your hand quickly transmit that information through the nervous system to the central nervous system. After processing an electrochemical signal up through the spine, the sensory signal is eventually received in the human brain. The brain takes action and, in this case, initiates an instinctual retraction of the hand.