In simple terms, what is the cause of brownian motion?? THANK YOU... THANK YOU!

Hey! Ya asked for "simple"? Boy! Have I got "simple".
To see a illustration of Brownian Motion click on:

http://www.math.rutgers.edu/~sontag/338/brownian-applet.html

Although that is a nice link, it does not explain that the cause of the Brownian Motion is the thermal agitation of the liquid's molecules.