Stalactite and stalagmite that a fourth grader can understand?

Answer 1

I hope my answering this does not make me party to some Fourth Grader's attempt to find another to do his or her homework, then plagiarise it...

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Rain water becomes slightly acid by dissolving carbon dioxide from the atmosphere into it. This Carbonic Acid is strong enough to dissolve limestone, whose main constituent is Calcium Carbonate (CaCO3). This is the primary way in which caves form in limestone hills in the first place.

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As the carbonic acid leaches through the joints in the limestone above the cave, it dissolves more of the CaCO3. When this solution emerges through the cave roof, some of the carbonate precipitates as its crystalline form, Calcite. So, drop by drop, crystal by crystal, a stalactite grows on the roof, and a stalagmite grows on the floor beneath.

(With apologies for the inexplicable and irreversible change of font size in that last paragraph!)

Answer 2

Stalactites are like icicles hanging from the ceiling of a cave, formed from minerals dripping down and leaving behind a cone-shaped deposit. Stalagmites are like upside-down icicles growing up from the cave floor, formed when minerals accumulate below and create a cone-shaped projection upwards. So, stalactites hang tightly from the ceiling, while stalagmites grow mightily from the ground up!