This is really amazing. This plant can actually play dead when it sense danger. The links below have a video that shows what i mean… (need quicktime).
Here is an excerpt from the site:

The leaves of Mimosa have the capability to display thigmonasty (touch-induced movement). In the sensitive plant, the leaves respond to being touched, shaken, heated or rapidly cooled. The speed of the response depends on the magnitude of the stimulus. Hitting the leaf hard with the flick of a finger will cause the leaf to close in the blink of an eye whereas a gentle touch or modest heat source applied to leaflets at the tip of a leaf will result in a slower response and the propagation of the stimulus along the leaf can be observed.

[Mimosa subtle reaction]

[Mimosa strong reaction]