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Why do crystals have flat sides?

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What else makes you wonder?

How can one impurity change a crystal’s shape?

An impurity can block some growing faces more than others.

Why do snowflakes keep six directions?

Compare water ice’s repeating molecular arrangement with its outer branches.

Why can a cube grow hollow-looking steps?

Sometimes edges and corners advance faster than the middle of a face.

ages 8–11If you dissolve a whole sugar cube in water, does the water weigh more? Open ages 8–11Why does hot cocoa mix in fast but cold milk leaves clumps? Open ages 8–11Can you keep stirring sugar into water forever? Open
After you watchWhy do crystals have flat sides?

The short answer

Crystals have flat sides because their particles occupy repeating internal arrangements. Some outer planes are especially stable or grow slowly, so they remain visible as facets.

Try this next

  • How can one impurity change a crystal’s shape? An impurity can block some growing faces more than others.
  • Why do snowflakes keep six directions? Compare water ice’s repeating molecular arrangement with its outer branches.
  • Why can a cube grow hollow-looking steps? Sometimes edges and corners advance faster than the middle of a face.

Now you — bend it

  • What if How can one impurity change a crystal’s shape?An impurity can block some growing faces more than others.
  • What if Why do snowflakes keep six directions?Compare water ice’s repeating molecular arrangement with its outer branches.
  • What if Why can a cube grow hollow-looking steps?Sometimes edges and corners advance faster than the middle of a face.

Can you prove it?Flat faces echo ordered internal planes. — Change one variable at a time, predict first, record the visible outcome, and compare repeated trials.

Design your own test:Choose one new value without copying the book’s hidden case. Predict the outcome, then explain what evidence would change your mind.

Explain it to a 6-year-old: Crystals have flat sides because their particles occupy repeating internal arrangements. Some outer planes are especially stable or grow slowly, so they remain visible as facets.

The whole story

How it works

A crystal lattice repeats in specific directions. As particles attach, complete planes can line up into flat faces. Different materials have different lattices and surface energies, so they do not all make the same shape.

What people get wrong

A crystal is not flat-sided because someone cut it, and fast growth does not automatically make it noncrystalline. Growth conditions can produce steps, hoppers, twins, and clusters while internal order remains.

The catch

Temperature, solvent, impurities, defects, crowding, and growth rate can change the visible crystal habit even when the lattice stays the same.

Questions kids ask

Are all crystals perfect blocks?

No. Impurities, growth rate, surface energies, damage, and nearby crystals can change the visible habit.

Does fast growth erase crystal order?

Not necessarily. Rapid salt growth can form hopper-shaped or interconnected cubes rather than a noncrystalline lump.

Talk about it

  • Ask for a prediction before opening the hidden experiment: “Which rim will many complete layers build?”
  • Ask which visible change was the cause and which was the measured outcome.
  • Ask what single variable should change in a fair follow-up test.

For grown-ups

Macroscopic facets reflect anisotropic lattice structure, surface free energies, and growth kinetics. The Wulff construction describes an equilibrium shape; real habits are often kinetic.