Why do hills and mountains that seem very solid in dry weather develop major landslides after prolonged rains? Build a sandcastle. After you have it shaped firmly, pour some water on it. Pour the water slowly and gently. Keep pouring until the sand can absorb no more water. What happened at first? What happened finally? How can you compare this to rainfall and mountains?
Natural Science, http://server2.greatlakes.k12.mi.us/explorer/GLCbrowse/Natural^Science/index.html