Stalactites are the long rock columns that grow from
the roofs of many caves. Stalagmites are similar columns
that grow from the cave's floor. Water from rain or other
sources migrates from the land surface through the
dirt and rock absorbing various minerals along the way.
Eventually this water finds its way to a cave ceiling where
it collects and starts to drip. Before the water has time to
drip to the floor is loses some of the minerals that it has
absorbed. Over a long period of time these minerals build
up and form a stalactite. After the water drops to the floor
more of the minerals are left behind forming the stalagmites.
Stalactites and stalagmites vary greatly in color and rate of
growth depending on they types of minerals the water has
collected. The easiest way to remember which one is
which is by the way the words sound.
Stalactites are hanging tightly from the ceiling.
Stalagmites are mighty hard and it will hurt if you trip over one.
It is important that you not touch them or break them off to keep
as souvenirs. They take many years to form. Touching them
will disrupt their growth or possibly even stop them from growing.
If you remove them then others will be deprived of being able
to see their beauty.

It is possible to simulate the growth of these formations.
Fill two jars with very warm water. Add as much baking
soda or Epsom salts to each jar as will dissolve. Place a
saucer between the two jars, which are about a foot apart.
Dip one end of a piece of absorbent (cotton, wool - some
synthetics won't draw the solution) yarn, thick thread or string
into each jar. The ends should be weighted with washers,
nails or what have you to keep them in the jars. Let it hang in
the middle, over the saucer. Capillary action should draw the
baking soda solution up through the yarn, where it will drip
onto the saucer. Over the course of several days, the dripping
water will deposit the baking soda, forming a tiny stalactite
and stalagmite. Eventually these may join to create a single
column, as in an actual cave. Epsom salts generally take
longer, but yield more variety of shapes.