Psychologists split what happens when you see a face, hear a song or taste a lemon into two jobs.

• Sensation is the first job. Specialised cells called receptors pick up a form of physical energy (light, sound waves, pressure, chemicals) and turn it into electrical signals the nervous system can use. That conversion is called transduction. In the eye, the receptors are the rods and cones of the retina. There are about 120 million rods and about 6 million cones in each eye.
• Perception is the second job. The brain organises and interprets those signals, so that a pattern of light and dark becomes "my friend, ten metres away, waving".

Sensation alone is a swarm of signals. Perception gives it a meaning.