To show how soil is formed from rocks
Carefully heat a piece of glass in a flame and then plunge it into cold water. The unexpected cooling of the glass causes it to contract unevenly, and it cracks. Heat some rocks very hot in a fire and then pour cold water on them. The rocks will often break up both when heated and when cooled. One of the stages in the formation of soil is the breaking up of rocks under dissimilarity of temperature.