The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

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.
Proper amount of water is the key to （） sand castles.