This is a cunning little game that can be fascinating.  It will especially appeal to those of us who enjoyed building dams in streams as children.

"Falling Sand Game" is, technically, it's a "Java Applet" but let's not dwell on that.  Click here to go to the site.  It looks a bit like this:

It starts with four continuously falling steams of particles that represent sand, water, salt and oil, and behave a bit like them. You use your mouse to add things to the picture to change what's happening to the falling streams.  When you start, if you hold down a mouse button and move the mouse, it builds a little wall that diverts the stream - try drawing a "U" shape to collect the particles until they overflow, for example.

Once you've got the hang of that, look under the picture, and you'll see that you can change what your mouse is doing. For example, click on "eraser" and you can rub out bits of your wall (and the stuff falls through) or burn it with fire, and so forth.  By putting these together, you can construct some very complex structures and systems. Do have a go - it will pass a worrying amount of time very easily.  You can also alter the flow speed and quantity, and other refinements.

There appears to be a large group of followers in the forums, but they don't sound like Oldies - and have some odd ideas.  I suggest we just concentrate on making pretty pictures.