Students should describe these changes in their lab notebooks, as well as the amount of water they rained on the castle. Initially, the changes in the sand castles will be subtle. At some point the sand castle will slump, causing a landslide. When all of the sand castles have slumped, gather the class together to discuss their observations. Ask students how much water they had poured on the castle when it began to look like a liquid instead of a solid.
Gravity provides the energy for landslides, but water also plays a number of roles.
As more water is added to the sand castle during the student-generated rain storm, the pore spaces fill with water and the force of the water actually pushes the sand grains apart, causing a landslide. Dry materials, such as sand, have a threshold of slope stability related to gravity and the cohesion of the material. Known as the angle of repose, this threshold limits the height of the castle for a given volume of sand. Extension Discuss how the sand castles could be designed to prevent them from slumping. Ask students what methods they have observed of preventing walls from slumping.