Stalactites and stalagmites are cave formations made when water dissolves overlying limestone and redeposits calcium carbonate. The National Park Service says stalactites form along ceilings and hang downward, while a mound-shaped stalagmite grows where that dripping water hits the floor.
A memory aid from Mammoth Cave National Park is that stalactites hang “tight” to the ceiling and stalagmites push up with all their “might.” If the two keep growing until they meet, they form a column from floor to ceiling.