I first heard about the Touch-Me-Not plant from my mother's stories of her childhood. There were plenty of these where she grew up, and she had fun playing with them. But no matter how much she described the plant to me, I never could quite imagine how the phenomenon worked. Now, thanks to the internet, I can see it for myself, on video.
The Mimosa Pudica (derived from Latin for shy), commonly known as the Touch-Me-Not, is very true to its name. It obviously doesn't like to be touched, because running a finger along the leaf simply causes it to fold inwards. I'm not sure if touch really bothers the plant, but the phenomenon is very beautiful to look at. The exact reason for this peculiar behavior of the Touch-Me-Not is also unknown, but it is believed the plant uses this feature to thwart predators. The closing leaves supposedly scare them away. Apart from the stimulus of human touch, the leaves also close during night time. The plant has its origins in South and Central America, but it is actually found all around the world.
The closing motion of the leaves is what makes this plant so special. In school, we all learned plants are real organisms, but we rarely perceive them like that in real life. When the Touch-Me-Not starts to shrink and closes its leaves one by one, it's a dramatic moment in which you feel the plant is actually alive.
Touch-Me-Not is popular as an ornamental plant, but needs to be handed with caution. A plant that is so sensitive to touch can actually be poisonous if ingested. It is especially dangerous for children who find it to be an object of great fascination.
Strangely enough, the leaves of the Mimosa Pudica don't close up if you touch them right after they've opened up in the morning. I have no idea why that is, they're probably too hungry for some photosynthesis...