I don't often make a big deal out of the Google Doodles, but I found the Doodle currently in use to be especially intriguing for a few different reasons. Though Google has used interactive Doodles in the past, this one is (to my knowledge) more subtly sophisticated than the rest. At first glance, it may simply appear to be animated. However, a closer look shows otherwise.
Moving the mouse back and forth across the screen, I noticed that the flame changes color gradually. (I'm using a maximized Google Chrome window on my Ubuntu installation, but I suspect that it will work the same with any modern browser.) I am guessing that this somehow corresponds to the different colors of flame produced when different chemicals are burned, but I don't know for sure - chemistry is definitely one of my weaker areas of physical science.
Also, moving the mouse up and down heightens or shortens the flame correspondingly; due to the increase or decrease of heat that this would produce in a real-life setup of a system like the one depicted, the overall activity of the system shown in the Doodle increases with the larger flame, and decreases with the smaller flame. For me, it was initially the blue liquid moving from place to place through the coiled structure that caught my attention, and helped me realize this was actually a more interactive Doodle, rather than a sophisticated-but-static animation, as I remember the one for Rube Goldberg as being (though now I question that recollection).
That's all for now; awesome stuff, keep it up, Google!