Why do the leaves of touch-me-not plant close when they are touched?

The Makahiya plant, more commonly known as the TickleMe Plant moves for the following reasons:

Some scientists think that the TickleMe Plant's movements may be the way that the plant protects itself from animals that attempt to eat it. An insect or other animal may touch the TickleMe Plant and the plant will begin to close its leaves and lower its leafstalks. When the TickleMe Plant moves, this may scare the animal, causing the animal to move away from it and go on to a less active plant. It could be that when the TickleMe Plant closes its leaves and lowers its leafstalks, its thorns become more exposed. Showing off its thorns, may be another way for the TickleMe Plant to warn plant eating animals to stay away.