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Why is it when you build a sand castle without water it falls and when you include water it stays up?
Question Date: 2013-02-11
Answer 1:

That's a fun question. A lot of things stick together with water - sand and dirt and cornstarch and bits of paper, for example. Those things all love water. The scientific name for that is 'hydrophilic' - 'hydro' for 'water,' and 'philic' for 'loving.' So the water pulls the things together.

I can think of 2 things that don't stick together with water. If you had gravel instead of sand, then the pieces of gravel would be too heavy for the water molecules to hold them together. And if you had little bits of styrofoam, maybe from shredding a styrofoam cup, the water wouldn't hold them together. That's because styrofoam molecules don't like water. The scientific name for that is 'hydrophobic' - 'phobic' means 'afraid of,' so 'hydrophobic' means 'afraid of water.' Of course molecules and sand and styrofoam don't really love or hate anything. They just interact with water in ways that pull the water molecules toward them or push away the water molecules.

Keep asking questions! Best wishes,

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