How do stalactites and stalagmites grow?

I included a science experiment that would cover science state standards for my 3 different levels of students, 6th, 7th and 8th graders.  I incorporated Ed puzzle and then a hands on activity that we had to watch over for a few days. We waited to learn how salt crystals grew from the bottom of the box up towards the top from the droppings that came from the strings at the top of the box. Each student had their own marked string to watch as it grew salt crystals that symbolized stalactites and stalagmites.