Needed: A roll of string
Takes about 30-50 sec per person.
People-pupils standing in a circle, the teacher helps to spread-divide the string across the circle here and there, people holding the string a web appears.
Then the person holding the roll of string starts speaking (any topic can be used) as long as reaching another person.
Sometimes some "crawling" and "climbing" needs to be done to get the string going... :)