Certain set lelation,such as set membership ,subset relation ,and set intersection.A piece of clay shapped like a doughnut (as in1)cannot be made into a piece of clay shaped like a disk(as in 2)without folding it onto itself,wich is against the rules. The doughnut can,however,be pulled around and pushed in so as to take on the shape of a cup without tearing or folding onto it self.lf the following figurs is drawb on a piece of balloon,no matter how it is pushed or pulled,there will always be exactly one point of intersection.