Only geologists are likely to use these terms often, but others have difficulty in remembering which hangs down from the roof of a cave and which rises up from the floor. Both stalactite and stalagmite refer to deposits of calcium carbonate formed by the dripping of water. A stalactite is a column that hangs down; one that builds up from the floor is a stalagmite. Suggestion for remembering: associate stalactite with icicle. If that doesn't help, think of mite and mire.
Dictionary of problem words and expressions. Harry Shaw. 1975.