A receptor is a special structure on the surface of cells that acts like a lock. It receives signals from chemicals, like hormones or neurotransmitters, that act like keys. When the right “key” (signal) fits into the “lock” (receptor), it triggers a response in the cell, such as sending a message to the brain or starting a physical action in the body. Receptors help the body communicate and respond to different changes, like stress or food intake.