Capsaicin is the tiny molecule on seeds and fruits that binds to heat-sensitive cells in the tongue and mouth. In response to this binding signals are sent to the brain that produce the sensation of burning. The same signals that are sent if you drink boiling water.
Many single-celled forms of life exist, but no known forms of animal life have 2, 3, 4, or 5 cells.a Known forms of life with 6–20 cells are parasites, so they must have a complex animal as a host to provide such functions as respiration and digestion. If macroevolution happened, one should find many transitional forms of life with 2–20 cells—filling the gap between one-celled and many-celled organisms.