At 11.00 am, following an online message calling for a protest against longer school days, thousands of pupils at schools across the country walk out of their lessons at the same time.

Earlier, Sofia told a friend that she had decided to remain in her lesson. When most of her classmates stand up and leave, she abandons that decision and goes with them. At one school, three pupils begin kicking over rubbish bins. The pupils nearest to them copy, and soon bins are being kicked over across the playground, frightening younger pupils.

Explain how the source shows collective behaviour.