Small games that simulate real-world physics are just fantastic - I already wrote about a very similar game but this one has far more options :-))

"This sandbox contains a variety of elements which the player can scatter at a mouse-click. Ice, water, fire, the titular “powder” (which serves as the sand). The heart of Powder Game is the way these elements interact. Water that touches ice will freeze. Ice that touches fire will melt. Drop a seed onto some powder and a plant will sprout. Water the plant and it will grow. Touch a flame to it and it will catch fire and burn.

The other key element that defines Powder Game is wind. Powder stacks in neat piles, but a click of the mouse (a right click by default, but this can be changed) sends it spiralling into the air. Currents coalesce, rub against each other, create eddies in the air. Fire creates wind, as does exploding gunpowder. Wind turns ice into snow, creates rivers of particles in the air. When the background effect is set to “BG-shade,” it becomes entrancing to watch."

There is even a whole website around those physic games:

Source 1: http://dan-ball.jp/en/javagame/dust/
Source 2: World of Sand-old article
Source 3: http://www.fun-motion.com/