What is a stalactite?

Stalactites form inside caves when water carrying dissolved minerals, chiefly calcium carbonate, drips slowly from the ceiling. As each drop hangs, a little carbon dioxide escapes and a tiny amount of calcite is deposited, gradually building a hollow, downward-growing spike over very long periods. A matching formation called a stalagmite grows upward from the floor where the drips land, and if the two eventually meet they form a column. Stalactites grow extremely slowly, often just a fraction of a centimetre per century, so large ones are thousands of years old. A simple way to remember the difference is that stalactites cling tight to the ceiling while stalagmites might reach the ceiling one day.