How do stalactites and stalagmites form inside caves? In nature these icicle-like shapes grow over thousands of years. You can speed up the process on your kitchen table with Epsom salt and yarn. You dissolve Epsom salt in two jars of hot water. Then you drape a piece of yarn between the jars with a plate below. Over several days the salt water creeps along the yarn and drips down. As the water evaporates it leaves behind crystal deposits. You watch a tiny stalactite grow from the yarn and a stalagmite build up on the plate.