The smallest parts of soil are sand and silt and clay. Those small parts join to make larger parts we call "clumps" or "aggregates". The clumps are made when sand and silt and clay stick together. The humus and clay and minerals in the soil are like glue. The glue sticks the sand and silt and clay together and makes clumps. The clumps make shapes by themselves. Some soils have small round clumps. Other soils have large, hard and flat clumps. The soil with small round clumps is best because it lets in air and water. A little glue is best. If the soil has only a little glue there will be space for water and air and the soil will be soft. If the soil has too much glue the soil will be hard. If the soil has no glue, there will be no space in the soil for air and water. A soil with no spaces is not healthy. Worms in the soil make a slippery glue. When worms make holes in the soil they leave some glue in the soil. Plant roots also make spaces in the soil. When the roots die they leave holes in the soil.