Below a Sandpile applet. Left click on the grid to add 1, middle click to subtract 1, right click to switch squares to sinks and back. The identity is obtained by pushing the Clear and Reduce buttons.
No Java? Perhaps install or enable Java, or use a different browser?
Colors: sink: black, 0: white, 1: yellow, 2: orange, 3: red, 4: magenta, 5: blue, larger: lighter blue.