Your teacher will provide several examples of granite, basalt, and obsidian rock. Look at them with a hand lens, and compare the different rocks to each other. Look for the
crystals in each rock.

You will see that granite contains large crystals, basalt contains very tiny crystals, and obsidian has no observable crystals.

Why is that? Rock is rock--it should all be the same. No?

Let's see if we can do a little experiment and find out how rocks are different, and also learn what is this about crystals.

We will prepare solutions of powdered alum in two test tubes. Put a spoonful of powdered alum and some water into each test tube. Shake the tube to dissolve the alum. (Instead of alum, we could use sugar, salt, Epsom salt or borax.)

Your teacher will boil the solutions over a candle flame, and add another spoonful of the solid substance to each test tube. (These solutions represent magma in the earth.)

Now hang small nails from strings, and suspend the nails in the test tubes. Set one test tube in a holder, and allow it to cool slowly.

Place the second test tube upright in a bowl of ice cubes to cool it quickly.

In twenty-four hours you will be able to see differences in the formation of crystals -- there will be large crystals in the test tube that cooled slowly, small crystals in the test tube that cooled quickly.

Make an analogy to rocks -- rocks that cool slowly contain large crystals; rocks that cool quickly contain small crystals.

Order your rock samples by the speed at which they probably cooled.