The scenery is constructed when each spectator holds up a coloured piece of paper. Emergent behaviour is an interesting phenomenon that can be observed in natural systems. We define emergent behaviour as being the behaviour of a system that is the product of interaction between smaller sub-systems. The emergent behaviour is of higher complexity than the sum of the behaviours of the smaller sub-systems. The reason that behaviour of higher complexity than the sum can emerge is the interaction between the sub-systems.
Emergent behaviour is known from flocks of birds, schools of fish and herds of land animals.
When observing a flock of birds, we will notice that there is no apparent leader in the flock and there appears to be no central control of motion. The motion of the flock might seem complex and at times random, but on the other hand, it also appears synchronous. The motion of a flock of birds is an example of emergent behaviour and can be modelled as such. The three rules are local in the sense that a boid only has knowledge about nearby boids and there is no global knowledge like size or centre of the flock. For instance, Flock Centering is achieved by having boids to perceive the centroid of nearby boids only.