Paragraph Two 　A sand castle is made of a network of sand grains glued together by very thin bridges of water. Without any water, sand flows; a perfectly dry sand castle would collapse into a heap. Too much water, on the other hand, creates sand soup

Paragraph Two 　A sand castle is made of a network of sand grains glued together by very thin bridges of water. Without any water, sand flows; a perfectly dry sand castle would collapse into a heap. Too much water, on the other hand, creates sand soup. But embedded in a matrix of water molecules, sand particles stay in place, somewhat like molecules in solid substances. 37. Proper amount of water is the key to b____ sand castles.

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